The United Nations Security Council is to meet on Friday to discuss the situation in the rebel-held Syrian province of Idlib.

The announcement was made by Nikki Haley, the US Ambassador to the UN.

The UN has warned that an assault by Syrian goverment forces there could spark a humanitarian catastrophe.

Government troops have been massing, before an expected offensive to retake the region.

Earlier, US President Donald Trump warned Syrian President Bashar al-Assad and his allies, Iran and Russia, not to "recklessly attack" Idlib, warning that hundreds of thousands of people could be killed.

The northern province and surrounding areas are the last major enclave held by insurgents fighting Mr Assad, who has been backed by both Russian and Iranian forces in Syria's seven-year-old civil war. 

"The Russians and Iranians would be making a grave humanitarian mistake to take part in this potential human tragedy. Hundreds of thousands of people could be killed. Don't let that happen!" Mr Trump wrote in a tweet.

The United Nations peace envoy to Syria called on the Russian and Turkish presidents to urgently speak to each other to help avert a "bloodbath" in Idlib.

Staffan de Mistura appealed to "President Putin and to President Erdogan ... to make a telephone call," even before they are set to meet their Iranian counterpart in Tehran on Friday.

"Let's try to avoid that the last probably major battle of the Syrian territorial conflict ... ends in a bloodbath," he told reporters in Geneva, insisting Russia and Turkey held "the key for the soft solution to the Idlib issue."

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Yesterday, Iran called for militants to be cleaned out of Idlib, as it prepared for talks with Syria and Russia about the fate of the province.

Special report: Syria's seven-year-war

US Secretary of State Mike Pompeo and Turkish Foreign Minister Mevlut Cavusoglu spoke today about the Syrian conflict.

Both men agreed the Syrian government's expected military offensive in Idlib would be "an unacceptable, reckless escalation," State Department spokeswoman Heather Nauert said.

Meanwhile, Iranian Foreign Minister Mohammad Javad Zarif visited Damascus to discuss plans for an upcoming summit between the leaders of Iran, Russia and Turkey, which Tehran will host on 7 September to discuss Idlib, Iran's Fars news agency reported.

Turkey, which has long supported anti-Assad rebels, has cooperated with Russia and Iran on talks over Syria in recent years and has troops in the Idlib region on an observation mission.

Last week, Iran's defence minister travelled to Damascus and signed an agreement for defence cooperation between the two countries with his Syrian counterpart.

With rebels defeated in most of Syria, an offensive on Idlib could prove to be the last big battle of the Syrian civil war.