US President Donald Trump has warned Russia, Syria and Iran against killing civilians in Syria's Idlib province and says Turkey is working hard to stop the "carnage".

"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," he said in a tweet on Thursday.

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.

Russia, Syria, and Iran are killing, or on their way to killing, thousands of innocent civilians in Idlib Province. Don’t do it! Turkey is working hard to stop this carnage. — Donald J. Trump (@realDonaldTrump) December 26, 2019

The White Helmet rescue organisation said 12 people, including six children and three women, were killed on Tuesday in Idlib. Ten of the victims lost their lives after a Russian airstrike targeted their camp, the group said.

Nine other areas were targeted by 15 airstrikes and over 56 artillery shells, it said.

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 unravelling of late.

AFP

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.

Mr 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 defence system.

Meanwhile, Russian military police have taken control of a base near the Syrian city of Raqqa that was controlled by US forces until a few days ago, the TASS news agency reported on Thursday.

READ MORE Donald Trump orders withdrawal of up to 1,000 US troops from northern Syria

Russian forces entered the nearby city of Raqqa, the former de facto capital of the Islamic State caliphate, earlier this month as Moscow rushed to fill a vacuum created by Mr Trump's decision to pull troops from northern Syria.

The base, a former school in the village of Tal Samin, is in a strategic area at a crossroads that links the city of Raqqa with central Syria and its northern regions, the state news agency said.

EPA

It cited Arman Mambetov, a Russian military policeman, as saying that he and colleagues would begin patrolling the surrounding area from Thursday.

The Russian defence ministry's Zvezda TV channel showed two Russian military policemen running up the Russian flag at the base and a column of Russian military police vehicles entering.