• As Russia warns US its fighter jets are now potential target in Syria

The US student held in captivity for more than 15 months in North Korea has died a week after returning home.

Otto Warmbier, 22, was serving 15 years hard labour, accused of attempting to steal a propaganda sign from a hotel.

He was returned to the US last Tuesday, with North Korea saying it was on humanitarian grounds.

North Korea said he had been in a coma for a year after contracting botulism but his family say he was subjected to “awful torturous mistreatment”.

A team of US doctors have also disputed North Korea’s version of events.

Mr Warmbier had suffered severe brain damage, and was medically evacuated from North Korea on 13 June to a hospital in his home city of Cincinnati, Ohio. It is unclear how he fell ill.

A statement from the family on Monday said: “It is our sad duty to report that our son, Otto Warmbier, has completed his journey home. Surrounded by his loving family, Otto died today at 2:20pm.”

They said the student had been “unable to speak, unable to see and unable to react to verbal commands”.

“The awful torturous mistreatment our son received at the hands of the North Koreans ensured that no other outcome was possible beyond the sad one we experienced today.”

In the meantime, the threat of direct Russian-American confrontation in Syria escalated on Monday after Moscow said it would treat any plane from the US-led coalition flying west of the Euphrates river as a potential target.

Russia said it was responding to US planes shooting down a Syrian air force jet on Sunday. The US said its planes had acted to defend US-backed forces seeking to capture the Islamic State capital of Raqqa in north-east Syria.

It was the first such US attack on a Syrian warplane since the start of the country’s civil war six years ago.

Russia’s deputy foreign minister, Sergei Ryabkov, said the US strike “has to be seen as a continuation of America’s line to disregard the norms of international law.

“What is this if not an act of aggression? It is, if you like, help to those terrorists that the US is fighting against, declaring they are carrying out an anti-terrorism policy.”

The Russian foreign ministry also said it would respond to the attack by suspending its communications channel with US forces, which is designed to prevent collisions and dangerous incidents in Syrian airspace.

The top US general, Joseph Dunford, sought to play down the repercussions of the incident, insisting the hotline established eight months ago between US central command in Qatar and its Russian equivalent in Syria was still open and functioning on Monday morning and would be used try to defuse the situation.

“I’m confident that we are still communicating between the coalition operations centre and the Russian operations centre,” Dunford, chairman of the joint chiefs of staff told the National Press Club in Washington. “I think the worst thing any of us could do would be to address this with hyperbole.”

He added: “I’m also confident that our forces have the capability to take care of themselves.”

The Russian military threatened to close the hotline, known as the “deconfliction channel”, in April after President Trump ordered a missile strike against a Syrian air base allegedly involved in a chemical weapons attack. However, Moscow did not fulfill the threat and the channel remained open.

BBC with additional report from Guardian