The UN Security Council will hold a meeting on Friday on the situation in Syria’s Idlib province, U.S. Ambassador to the United Nations Nikki Haley said on Tuesday, warning Syria’s government not to use chemical weapons ahead of a widely expected military offensive there.

Russian and Syrian planes pounded areas on the western edge of Idlib, the last major rebel stronghold, on Tuesday, according to The Syrian Observatory for Human Rights and a rebel source. A Syrian government minister also said the siege of Idlib would probably be resolved by force, according to Reuters.

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“This is a tragic situation, and if they want to continue to go the route of taking over Syria, they can do that,” Haley said at a news conference, referring to the government of Syrian President Bashar al-Assad as well as its allies Russia and Iran.

“But they cannot do it with chemical weapons. They can’t do it assaulting their people and we’re not going to fall for it,” she added.

The world’s chemical weapons watchdog has documented systematic use of banned munitions in Syria’s civil war, including nerve agent sarin and sulfur mustard gas. But the body, the Organization for the Prohibition of Chemical Weapons, has not assigned blame for the attacks.

“If there are chemical weapons that are used, we know exactly who’s going to use them,” Haley said.

Earlier this week, U.S. President Donald Trump warned Syria and its allies Russia and Iran against attacking the last major rebel stronghold of Idlib province in the country's northwest.

"President Bashar al-Assad of Syria must not recklessly attack Idlib Province," Trump wrote on Twitter. "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!" Donald Trump tweeted.