Sept. 4 (UPI) -- Syrian government warplanes struck the last rebel-held province of the country Tuesday.
The Syrian government's attack on Idlib, located in northwestern Syria, was assisted by the Russian military, which called the area a "cradle of terrorism," the BBC reported.
The attacks were carried out after the U.S. government urged the Syrian government, which is assisted by Russia and Iran in its civil war, not to attack the rebel stronghold where approximately 3 million people live.
"President Bashar al-Assad of Syria must not recklessly attack Idlib province," U.S. President Donald Trump said via Twitter on Monday. "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!"
On Tuesday, the White House released an official statement condemning the attacks on Idlib and threatened military force if "chemical weapons" are used.
"Let us be clear, it remains our firm stance that if President Bashar al-Assad chooses to again use chemical weapons, the United States and its allies will respond swiftly and appropriately," the statement read.
More than 3 million people live in the Idlib area and U.S. Gen. Joseph Dunford, the chairman of the Joint Chiefs of Staff, told reporters that a "human catastrophe" could unfold if the Syrian government attacks the province.
The United States has approximately 2,000 troops in Syria.