President Donald Trump warned Syria and its allies Russia and Iran on Monday 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!”

Hours after the tweet, Russian planes struck parts of western Idlib, according to an activist media group and Syrian rescue group the White Helmets. The White Helmets said rescues were underway amid “intensive airstrikes.”

The President’s warning comes just days after Secretary of State Mike Pompeo issued a similar warning to Russia’s foreign minister, Sergey Lavrov, saying an attack on Idlib was something that Syria and Russia “agreed not to permit,” and that the US “sees this as an escalation of an already dangerous conflict.”

US officials have grown worried that an assault on Idlib could involve the use of chemical weapons if the rebels are able to slow regime advances.

Syrian President Assad’s regime has moved armed helicopters closer to Idlib in recent weeks, according to two defence officials.

The US is concerned they could eventually be used to launch another chemical attack as well as a conventional assault.

The day before Pompeo’s warning, Lavrov told Western nations not to “play with fire” over Idlib and claimed the White Helmets, a group of unarmed volunteer rescue workers, were preparing to stage a chemical strike to blame on the Syrian government as a pretext for military action.

The Pentagon responded Friday, saying, “Russia has recently launched a concentrated disinformation campaign to discredit the United States and international partners and allies” and calling Lavrov’s allegation about preparations for a staged chemical strike “absurd.”

CNN reported Friday that US intelligence and military targeting experts have created a list of Syrian chemical weapons facilities that could be struck if Trump decided to order a new round of airstrikes in the country.

So far, a decision to take action has not been made, but one administration official with direct knowledge of the situation told CNN the military “could respond very quickly” if Syria launched a chemical weapons attack.

About 6.2 million Syrians have been displaced since the conflict began in 2011, with a further 6.3 million having fled abroad as refugees, according to a recent UNHCR report.

Idlib is home to some 3 million Syrians, according to United Nations estimates, and 70,000 fighters from multiple factions, including former al Qaeda affiliate Hayat Tahrir al-Sham. The heavy concentration of residents and fighters, analysts say, makes the province a powder keg.

Half of Idlib’s population are displaced people, bused to the northwestern province from former rebel areas across the country. The area is densely populated, with many civilians flocking to camps near Syria’s border with Turkey as they brace for an imminent attack.