• As Russia accuses Ukraine of attempted Crimea ‘incursions’

Fifteen of the last 35 doctors in rebel-held eastern Aleppo have written a letter to Barack Obama with an urgent plea for intervention to stop the bombardment of hospitals in the besieged city by the Russian-backed Syrian air force.

“We do not need tears or sympathy or even prayers: we desperately need a zone free from bombing over eastern Aleppo to stop the attacks, and international action to ensure Aleppo is never besieged again,” the doctors wrote.

Their letter came as the Russians proposed daily three-hour ceasefires, and the UK circulated plans at the UN in New York for a ceasefire in which humanitarian relief was implemented by impartial actors, rather than the Syrian and Russian military.

The Russian defence ministry said on Wednesday there would be daily three-hour ceasefires in Aleppo starting from Thursday to allow humanitarian convoys to enter the city safely.

Speaking at a briefing, Gen Sergei Rudskoi, a senior Russian defence ministry official, said the pause in fighting would take place from 10am to 1pm local time.

Rudskoi also said about 7,000 fighters from Jabhat Fatah al-Sham – the militia previously known as the al-Qaida affiliate Jabhat al-Nusra – had been massing south-west of Aleppo for the past week and were still being joined by new fighters. He said the militants had tanks, infantry fighting vehicles, artillery and vehicles with weapons mounted on them.

The UN humanitarian chief, Stephen O’Brien, said that he was willing to consider the Russian plan, but added that getting sufficient aid into Aleppo would require a 48-hour pause in the fighting and an open single-carriageway road.

“When we’re offered three hours, then you have to ask: what could be achieved in those three hours?” he said. “Is it to meet the need or will it only just meet a very small part of the need?

The US ambassador to the UN, Samantha Power, has made repeated pleas for 48-hour ceasefires to allow aid into the bombarded city, and for some of the most seriously injured to be removed. A meeting of the UN security council on Tuesday failed to produce an immediate agreement on a ceasefire. Further talks are expected in Geneva on Thursday.

The joint letter written by the doctors urged the US president “to act now to stop the bombs that continue to fall on the city and ensure they are never held under siege again”.

It has been claimed that one medical facility is being attacked every 17 hours, and doctors are being forced to make appalling decisions to let children die due to a shortage of blood, medical supplies and more complex scanners.

Some doctors refused to sign the letter because they had no wish to make any more appeals to the west.

There seems little doubt in the minds of western diplomats that the Syrian forces have decided to target the remaining medical facilities in the eastern part of the city in an attempt to terrorise the remaining 250,000-300,000 citizens into submission, starvation or exodus. The six facilities hit in and around Aleppo included a paediatric clinic inside the city where four infants died after their oxygen supply was cut.

It has not been possible to verify the names of all the doctors listed in the letter, but their account tallies with evidence given by US doctors to the UN after a working visit to Aleppo’s hospitals in the past fortnight.

Aleppo is the second-largest city in Syria and has a symbolic importance to the revolution; its fall would confirm the extent to which the president, Bashar al-Assad, had gained the military upper hand thanks to Russian air support.

In their letter, the Syrian doctors write: “For five years, we have faced death from above on a daily basis. But we now face death from all around. For five years, we have borne witness as countless patients, friends and colleagues suffered violent, tormented deaths. For five years, the world has stood by and remarked how ‘complicated’ Syria is, while doing little to protect us. Recent offers of evacuation from the regime and Russia have sounded like thinly veiled threats to residents – flee now or face what fate?

“Last month, there were 42 attacks on medical facilities in Syria, 15 of which were hospitals in which we work. Right now, there is an attack on a medical facility every 17 hours. At this rate, our medical services in Aleppo could be completely destroyed in a month, leaving 300,000 people to die.

“What pains us most, as doctors, is choosing who will live and who will die. Young children are sometimes brought into our emergency rooms so badly injured that we have to prioritise those with better chances, or simply don’t have the equipment to help them. Two weeks ago, four newborn babies gasping for air suffocated to death after a blast cut the oxygen supply to their incubators. Gasping for air, their lives ended before they had really begun.”

They warn Obama that “unless a permanent lifeline to Aleppo is opened, it will be only a matter of time until we are again surrounded by regime troops, hunger takes hold and hospitals’ supplies run completely dry.

“We do not need to tell you that the systematic targeting of hospitals by Syrian regime and Russian warplanes is a war crime. We do not need to tell you that they are committing atrocities in Aleppo,” the letter states.

The UK has circulated a draft UN resolution on the Aleppo crisis making it clear that any humanitarian solution cannot be run by the Russians or Syrians alone, but must instead be led by impartial humanitarian actors.

The draft resolution states: “Any proposed humanitarian initiatives for civilians to escape the fighting must be guaranteed by all sides and independently implemented and monitored, and all civilians should be guaranteed voluntary, free movement, including the right to choose their route and destination, if they choose to leave. Humanitarian aid must be delivered to the population wherever they may be, irrespective of whether they chose to leave or remain in Aleppo.”

The former Liberal Democrat leader Lord Ashdown urged UK ministers to end their summer silence and speak out against the carnage in Syria.

He said: “ The UK must act quickly and decisively to establish humanitarian access to Aleppo, and if necessary provide RAF plans to aid in air drops to the desperate, besieged citizens.”

In the meantime, Russia has accused Ukraine of trying to carry out armed incursions into Crimea – the territory annexed by Russia in 2014 after an unrecognised referendum.

The FSB intelligence agency said two attempted incursions had taken place over the weekend and a Russian soldier and an FSB employee had been killed.

President Vladimir Putin vowed “further security measures” in response to Ukraine’s “stupid and criminal” acts.

Ukraine’s president described the accusations as “preposterous”.

“Russian accusations against Ukraine of terrorism in occupied Crimea sound as preposterous and cynical as the statements of the Russian leadership about the absence of Russian troops in the Donbass [region of rebel-held eastern Ukraine],” Petro Poroshenko said.

“These fantasies are only a pretext for making more military threats against Ukraine,” he added.

Russian state TV has been running pictures from the FSB, showing rucksacks packed with explosives. Ukrainian “saboteurs”, sent by the defence ministry, were supposedly intending to use them in Crimea.

The claims of Ukrainian “terrorism” on the peninsula have sparked concern here that Moscow could respond with some kind of military action. The possibility has been discussed actively on social media, especially after President Putin’s comment that he will not just “pass over” the death of two Russians.

Others suggest that the incident will be used – or was engineered, depending on your viewpoint – to scupper the complex peace process in Ukraine.

Straightaway, Mr Putin said more internationally-backed talks, due in the next few weeks, had been proved “pointless”. So Russia has a reason to stall again on the Minsk peace process, maintain the unstable status quo in Ukraine and to blame Kiev for all of it.

Russia says one of those detained is Yevhen Panov. Initial reports suggest he was a former military volunteer fighting for Ukraine in the east of the country. However he has apparently more recently been associated with a charitable organisation.

Ukraine has managed to contain the Russian-backed separatists in eastern Ukraine and, in purely military terms, it is hard to see what Ukraine has to gain by provoking its larger, much more powerful neighbour. For all the talk about rapprochements and peace in eastern Ukraine, there has been very little concrete progress. Fighting between Russian-backed separatists and the Ukrainians goes on every day.

Just two weeks before Ukraine celebrates 25 years of independence after the collapse of the Soviet Union in 1991, relations between Moscow and Kiev have taken another turn for the worse.

Kiev is preparing to celebrate the 25th anniversary of Ukrainian independence on 24 August, and officials have suggested Moscow may try to disrupt the celebrations.

Russia is preparing for a parliamentary election on 18 September.

The FSB said Ukrainian forces had intended to sabotage key infrastructure in Crimea.

“The aim of this subversive activity and terrorist acts was to destabilise the socio-political situation in the region ahead of preparations and the holding of elections,” the spy agency said.

It said the attempted incursions had been accompanied by “heavy fire” from Ukrainian territory, resulting in the death of the Russian soldier. The FSB employee died while trying to detain infiltrators, it said.

Bombs, ammunition and mines had been found in the area where “terrorist attacks” had been foiled, it said.

Mr Putin said there was no point in further negotiations with Ukraine and called on the US and European countries to put pressure on Kiev to change its behaviour.

About 9,000 people are reported to have been killed in the conflict in eastern Ukraine that followed the Russian annexation of Crimea.

The annexation, which was itself largely bloodless, followed a vote to secede at a referendum on self-determination on 16 March 2014 – a ballot which was not recognised internationally.

The US and European Union imposed sanctions on Moscow in response to its seizure of the territory.

Guardian with additional report from BBC