US President Donald Trump has called for the governments in Moscow, Damascus and Tehran to stop the violence in Syria's rebel-held province of Idlib.

"Russia, Syria, and Iran are killing, or on their way to killing, thousands" of civilians in the northwestern province, Mr Trump tweeted, adding: "Don't do it!"

Heightened regime and Russian bombardment has hit jihadist-held Idlib - the country's last major opposition bastion - since mid-December, as regime forces make steady advances on the ground despite an August ceasefire and UN calls for a de-escalation.

Nearly 80 civilians have been killed by air strikes and artillery attacks over the same period, according to the Syrian Observatory for Human Rights, which estimates that more than 40,000 people have been displaced in recent weeks.

On Tuesday, Turkey called for the attacks to "come to an end immediately," after sending a delegation to Moscow to discuss the flare-up.

Presidential spokesman Ibrahim Kalin said Ankara was pressing for a new ceasefire to replace the August agreement.

Mr Trump praised Turkey's efforts today, tweeting that Ankara "is working hard to stop this carnage."

Joint Turkey-Russia patrols near in Syria

Meanwhile, a leading aid agency has warned that an additional half a million people could be displaced in Syria's rebel held province of Idlib if the government's Russian backed offensive continues.

The International Rescue Committee in Syria said this could push the number of displaced people inside Idlib to over a million triggering a major humanitarian crisis.

The Committee said settlements housing those who were forced to flee their homes earlier this year have been over capacity for months.

A Turkish delegation travelled to Russia on Monday for talks on Syria, following reports that Russian-backed attacks there were forcing tens of thousands more Syrians to flee towards Turkey.

Turkish Presidential spokesman Ibrahim Kalin said on Tuesday that Russia would work to stop attacks in the northwestern Syrian region of Idlib after talks with the Turkish delegation in Moscow and that Ankara expected this promise to be kept.

The two sides also discussed Libya after Turkish President Tayyip Erdogan said last week his country would not remain silent in the face of "mercenaries" such as the Russian-backed Wagner group of private military contractors, supporting Khalifa Haftar's forces there.

Moscow has said it is very concerned about the prospect of Turkish troops being deployed to Libya.