WASHINGTON (AP) — Senior U.S. officials warned the Russian and Syrian governments Tuesday against chemical weapons use in Syria as forces allied with its President Bashar Assad prepare for an offensive on a rebel stronghold.

The United States "will respond to any verified chemical weapons use in Idlib or elsewhere in Syria ... in a swift and appropriate manner," State Department spokeswoman Heather Nauert told reporters in Washington.

Nauert said senior U.S. officials engaged with their Russian counterparts to "to make this point very clear to Damascus." She said the use of chemical weapons "will not be tolerated."

She said Secretary of State Mike Pompeo told Russian Foreign Minister Sergey Lavrov last week that Moscow — a military ally of Assad — would be held responsible.

The Syrian government is gearing up for an expected offensive in Idlib province, which is home to nearly 3 million people and has a large al-Qaida presence in addition to Syrian rebel groups.

President Donald Trump has twice carried out airstrikes in Syria in response to apparent chemical weapons attacks there. Trump said the strikes were intended to deter Assad from launching chemical weapons attacks again.

At the Pentagon, Defense Secretary Jim Mattis referred to those two airstrikes but offered no further information Tuesday on how the U.S. was responding to the situation, other than to cite the State Department's "recent active communication with Russia to enlist them in preventing this."

In April, the United States, France and Britain launched military strikes in Syria to punish Assad for an apparent attack using chlorine against civilians in the Damascus suburb of Douma. And in 2017 Trump authorized a barrage of Tomahawk cruise missiles to hit a single Syrian airfield in retaliation for Assad's use of sarin gas against civilians.

Assad has repeatedly denied his government has used chemical weapons.

Russia has accused Syrian rebels of preparing chemical attacks, which Moscow says the West will use to justify a strike against Syrian government forces.