"Russia, Syria, and Iran are killing, or on their way to killing, thousands if inocent (sic) civilians in Idlib Province. Don’t do it! Turkey is working hard to stop this carnage," Trump said in a tweet.

Syrian and Russian forces have intensified their bombardment of targets in Idlib, the last significant rebel pocket of Syria. Syrian President Bashar al-Assad has vowed to recapture it.

The leaders of Turkey, Russia and Iran agreed in Ankara in September to "de-escalate" conflict in Idlib after a months-long campaign that forced at least 500,000 civilians to flee. With diplomacy on a Syria peace settlement stalled, the de-escalation deal has been unraveling of late.

Turkish Presidential spokesman Ibrahim Kalin said on Tuesday that Russia will work to stop attacks in Idlib after talks with a Turkish delegation in Moscow.

Trump has sought a close relationship with Turkish President Tayyip Erdogan, even after Ankara's recent incursion against American Kurdish allies in Syria and Turkey's purchase of the Russian S-400 missile defense system.