U.S. President Donald Trump warned Syrian President Bashar al-Assad and his allies Iran and Russia on Monday not to “recklessly attack” Syria’s rebel-held Idlib province, warning that hundreds of thousands of people could be killed.

“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!” Trump wrote in a tweet.

Assad has vowed to defeat the opposition in its last refuge in the northwestern province if the rebels do not surrender to government rule.

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Idlib and the surrounding area is home to some 3 million people – nearly half of them already displaced more than once by the civil war. Tens of thousands of people fled to Idlib after surrendering in government offensives elsewhere, choosing to relocate to an opposition-held area rather than risk reprisals or forced conscription at the hands of the government.

UN officials believe an offensive on Idlib would trigger a wave of displacement that could uproot an estimated 800,000 people and discourage refugees from returning home.

Thousands of government troops and allied fighters have been massing in areas surrounding the province.

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

Trump has sought better relations with Russia since taking office in 2017 but the United States has been unable to rein in Moscow’s military and diplomatic support for Assad.

U.S. Secretary of State Mike Pompeo said Friday taht Washington views any government assault on Idlib as an escalation of Syria’s war, and the State Department warned that Washington would respond to any chemical attack by Damascus.

Earlier Monday, Iran called for militants to be “cleaned out” of Idlib, as it prepared for talks with Syria and Russia.

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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 Sept. 7 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 defense minister traveled to Damascus and signed an agreement for defense cooperation between the two countries with his Syrian counterpart.

“Syria’s territorial integrity should be safeguarded and all tribes and groups, as one society, should start the reconstruction process, and the refugees should return to their homes,” Zarif said.