WASHINGTON — The White House warned Syria's President Bashar Assad on Tuesday that if he chooses to use chemical weapons, the U.S. and its allies "will respond swiftly and appropriately," pushing back against an expected Syrian government offensive against an opposition stronghold.

The White House said in a statement that President Donald Trump was monitoring the situation in the rebel-held Idlib province, where it said millions of innocent civilians are under threat of an attack by Assad's government.

Trump has warned that an attack "would be a reckless escalation of an already tragic conflict," the White House said, risking the lives of hundreds of thousands of people. "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.

The U.S. has twice struck Syrian military installations in response to alleged poison gas attacks. The Syrian government denies ever using chemical weapons.

The Trump administration declaration came after the Kremlin criticized Trump's warnings against the expected offensive in the northern province, where Syria's rebels are holed up and have come under intense airstrikes.

Russia has been a key supporter of Assad and complained about militants in Idlib targeting its own facilities.

Trump on Monday sent a tweet warning the Syrian government and its allies against a "reckless attack" on the Idlib province. Kremlin spokesman Dmitry Peskov said the militants pose a terrorist threat, calling Idlib a "hornets' nest of terrorists."