A senior US official on Thursday urged Europe to support Turkey's military operation in Syria, where it has lost more than 50 soldiers trying to hold back a Russian-backed offensive on Idlib.
"One of the principles that the president (Donald Trump) and the US Congress share is that there should be a collective effort, not just of Turkey and the US but of our NATO allies, essentially the Europeans," the US special representative for Syria, James Jeffrey, told a conference in Istanbul.
"We are pressing the Europeans to contribute a great deal," he added.
The Syrian government, backed by Russian air power, has steadily retaken swathes of the province of Idlib, dominated by jihadists and Turkey-backed rebels, since launching its offensive in December.
Nearly one million people have fled their homes in the biggest displacement of the nine-year conflict.
Turkey fears another major influx of refugees, adding to the 3.6 million Syrians it already hosts.
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It announced a full operation against Syrian forces following an air strike that killed 34 of its soldiers on February 27.
Turkey has requested greater military support from its NATO partners, and has sought to pressure Europe by removing restrictions on migrants trying to leave its territory for the EU.
NATO has offered its solidarity to Turkey, but has yet to take concrete action.
"There's a Spanish Patriot (missile defence) unit right now deployed in Turkey at the Incirlik airbase, that's an example of things that NATO is actually doing and we want to see more actions like that," Jeffrey said.
He acknowledged the situation was complicated by Turkey's decision to buy a Russian missile defence system, the S-400, which was strongly opposed by NATO.
"We are looking for ways to work around it, but for the moment that is an issue," Jeffrey said of the S-400.
The a possibility remains of a direct conflict between Turkey and Russia in Syria, he added.
"We do not think there is a desire on either the part of Russia or Turkey... but Syria is the place were your best intentions get disappointed," he said.
"We are not sure who has done all the shooting on both sides over the last couple of weeks because you have military forces all mixed up. The fact that you have all those outside armies, all of those non-state armed actors, all in close proximity shooting at each other, leads to the danger area.