Turkish President Recep Tayyip Erdogan on Thursday called an urgent meeting after at least nine Turkish troops were killed in an air strike in Syria's Idlib region.

Russia is backing a devastating offensive by President Bashar al-Assad's regime in Idlib against rebel groups backed by Ankara.

Kay Bailey Hutchison, the US ambassador to NATO, said the Turks should see "who is their reliable partner and who isn't."

Turkey, a member of the Western alliance, defiantly began receiving deliveries last year of Russia's S-400 missile system despite US warnings and eviction from the F-16 fighter-jet program.

"I hope that President Erdogan will see that we are the ally of their past and their future and they need to drop the S-400," Hutchison told reporters in Washington.

"They see what Russia is, they see what they're doing now, and if they are attacking Turkish troops, then that should outweigh everything else that is happening between Turkey and Russia," she said.

Russia and Turkey agreed in 2018 to create a demilitarized zone in Idlib in an accord that has since fallen through.

The US says the S-400 system will hurt NATO by allowing Russia to improve its tracking of Western aircraft.

Turkey insists it will set up the S-400 in areas not frequented by US jets and it has not yet put the system into operation, giving Washington hope that Ankara may still change its mind.