Turkey must not conduct further military operations in north-west Syria, even if it has the right to defend itself, the White House said late on Wednesday.
Ankara in recent weeks has warned of a new ground offensive into Kurdish-controlled parts of Syria after a bomb attack in Istanbul last month, which it blames on Kurdish separatists based in Syria and northern Iraq. It faces growing international calls for restraint.
"We don't want to see military operations conducted in north-west Syria that are going to put civilians at greater risk than they already are, put in peril our troops and our personnel in Syria, or our counter ISIS mission," said White House national security spokesman John Kirby.
Turkey has increased its shelling and air strikes on northern Syria, putting the lives of thousands of civilians on the line.
The Syrian Democratic Forces (SDF), which helped to defeat ISIS in Syria, on Friday said it had stopped all joint counterterrorism operations with the US and other allies as a result of Turkey bombarding its area of control.
US officials confirmed the move.
Countering a new Turkish military operation would mean the SDF diverting its resources from protecting a prison holding ISIS fighters and fighting ISIS sleeper cells that are carrying out attacks in Syria, the SDF said.
Washington said it recognises that Turkey has a right to defend itself, especially against terrorism.
"We recognise the threat that the Turkish people are under, but we don't believe that ... this idea of military operations in north-west Syria is the best way to get at that threat," Mr Kirby said.
The US State Department said on Tuesday that Washington has made it clear to Turkey, publicly and privately, that it strongly opposes military action, including a potential land incursion in northern Syria.
“We remain concerned by escalating action in northern Syria, including recent air strikes, some of which directly threaten the safety of US personnel who are working to defeat ISIS," State Department spokesman Ned Price said.