US President Donald Trump on Thursday warned Russia, Syria and Iran against killing civilians in the Syrian province of Idlib and said that Turkey was working hard to prevent a "massacre."

"Russia, Syria and Iran are being killed or are in the process of killing thousands of innocent civilians in Idlib," Trump said on Twitter. Don't do that! Turkey is working hard to prevent this massacre. ”

Syrian and Russian forces have stepped up bombardment of targets in Idlib, the last major enclave under the control of the opposition in Syria. Syrian President Bashar al-Assad vowed to take him back.

The leaders of Turkey, Russia and Iran agreed in Ankara last September to "reduce escalation" in the conflict in Idlib province after a campaign that lasted several months, forcing at least 500,000 people to flee their homes.

As diplomatic efforts to reach a peaceful settlement to the conflict in Syria faltered, the de-escalation agreement has tumbled recently.

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

Trump seeks to strengthen his relationship with Turkish President Recep Tayyip Erdogan even after the recent Turkish incursion into Syria against the Kurds, US allies, and Ankara's purchase of the Russian S-400 missile defense system.