The US has expressed “deep concern” at Turkish air strikes that killed about two dozen Kurdish fighters in Syria and Iraq.
The US-backed Popular Protection Units (YPG), fighting against IS, said their positions were hit multiple times.
Turkey regards the YPG as linked to outlawed Kurdish separatists. Iraqi Kurdish Peshmerga, friendly to Turkey, also reportedly suffered fatalities.
The Iraqi government condemned the strikes carried out on its territory.
US state department spokesman Mark Toner said: “We are very concerned, deeply concerned that Turkey conducted air strikes earlier today in northern Syria as well as northern Iraq without proper co-ordination either with the United States or the broader global coalition to defeat IS.”
He added: “We have expressed those concerns to the government of Turkey directly.”
Iraqi government spokesman Saad al-Hadithi said: “The Iraqi government condemns and rejects the strikes carried out by Turkish aircraft on Iraqi territory.”
Turkish President Recep Tayyip Erdogan defended the air strikes, telling Reuters: “We are obliged to take measures. We must take steps.
“We shared this with the US and Russia and we are sharing it with Iraq as well. It is an operation that (Iraqi Kurdistan President Massoud) Barzani has been informed about.”
The number killed has not been confirmed but reports suggest about 18 YPG and five Iraqi Kurdish Peshmerga members died.
The Syrian Observatory for Human Rights, a UK-based monitoring organisation, said a dawn strike on Tuesday targeted Kurdish positions in Hassakeh province in north-east Syria, hitting a media centre and radio station.
A separate series of strikes hit a base near Sinjar in northern Iraq, close to the Syrian border.
Mr Erdogan said he regretted the death of the Peshmerga, saying it was “absolutely not an operation against [them]”.
The Peshmerga are the military forces of the autonomous region of Iraqi Kurdistan.
In a statement, the Peshmerga said that while the attack was “unacceptable”, it blamed forces from the Kurdistan Workers Party (PKK) in the area and called on them to withdraw.
The PKK is a Turkish-Kurdish rebel group that has been fighting an armed struggle against the Turkish government since the 1980s.
The strikes reflect how complicated the situation is in the battle against so-called Islamic State in Syria and northern Iraq.
The US is relying heavily on Kurdish forces on the ground but its ally, Turkey, views the YPG as an extension of the PKK.
A ceasefire between Turkey and the PKK ended last year and clashes have since claimed hundreds of lives on both sides.
A commander for the Kurdish forces in Syria called on its international allies to defend their forces from further attacks.
“We are asking the international coalition to intervene to stop these Turkish violations,” the unnamed commander told the AFP news agency.
“It’s unthinkable that we are fighting on a front as important as Raqqa while Turkish planes bomb us in the back,” he said, referring to the IS stronghold in northern Syria.
A US military commander met Kurdish fighters after the attack to show solidarity.