Russia on Tuesday again accused Syrian rebels of preparing a chemical attack which Moscow says will be used to justify a Western strike against Syrian troops.

Deputy Foreign Minister Sergei Ryabkov said Tuesday that the al-Qaida-linked Levant Liberation Committee is preparing the attack in the northern Idlib province, and that it would be filmed by first-responders known as the White Helmets, who Russia has accused of fabricating past attacks.

Also on Tuesday, senior U.S. officials warned the Russian and Syrian governments against chemical weapons use in Syria as forces allied with its President Bashar Assad prepare for an offensive on a rebel stronghold.

Everyone Wants to Get Iran Out of Syria. But No One Knows How to Do It ■ Syria, Turkey, Russia and U.S. to Square Off in Idlib With Millions of Civilians in the Crosshairs ■ Russia Deploys Fleet Off Syria While Claiming U.S. Preparing for Possible Strike

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.

Western countries and independent analysts say Syrian government forces have conducted several chemical weapons attacks over the course of the seven-year civil war. Alleged chemical attacks in 2017 and earlier this year led the U.S. to launch punitive strikes against Syrian forces.

Russia is a close ally of the Syrian government, which denies ever using chemical weapons. The Syrian government has been sending reinforcements toward Idlib for weeks ahead of an expected offensive against the last major rebel stronghold in the country.

Last week, Russian Maj. Gen. Alexei Tsygankov, who heads the center for reconciliation of warring parties in Syria, claimed that British special services were involved in plans for the alleged provocation.

That brought a heated denial Tuesday from Britain’s U.N. ambassador. “Even by the egregious standards of Russian propaganda, this is an extraordinary allegation,” Karen Pierce said during a Security Council session on the humanitarian situation in Syria. “It is wholly untrue.”

She said the claim was either aimed at disseminating “fake news” or “as a smoke screen for a possible impending attack by the Syrian regime, once again against its own people, in Idlib.”

“I repeat the statement that I, the French and the Americans made earlier about responding appropriately,” she added.

U.S. National Security Adviser John Bolton last week warned Syria’s government against using chemical weapons and vowed a tough response if it does.

Russia’s U.N. ambassador, Vassily Nebenzia, stood by the allegations, saying “if the Defense Ministry says something, then it says that based on concrete facts.”

“The Syrian armed forces do not have chemical weapons and have no plans to use them. There is no military need for that. We have stated that more than once. People in their right minds will not use means that are useless from a military point of view in order to trigger reprisals by three major powers,” he said.

Security Council diplomats said that during closed consultations, Pierce read the names of about half a dozen Syrian military commanders on the ground in Idlib and said they would be personally held accountable if there is a chemical attack. The diplomats spoke on condition of anonymity because the consultations were private.