Russia's foreign minister on Wednesday said that Moscow would respond to a U.S. "provocation" in Syria "proportionally and with dignity."

Sergey Lavrov warned the U.S. against launching a preemptive strike in Syria based on intelligence assessments that the regime of Syrian President Bashar Assad is planning a new chemical attack.

"They should not create excuses for more attacks against the Syrian army that is fighting terrorists," Lavrov added, saying terrorist groups could try to draw the U.S. into a strike against Syria by pinning responsibility for attacks against civilians on the Assad regime.

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The White House late Monday said that it had intelligence suggesting that Syrian forces were preparing for a chemical weapons strike, similar to one in April that killed dozens of civilians in the country's northern Idlib province.

White House Press Secretary Sean Spicer in a statement warned that the Assad regime would "pay a heavy price" if it proceeded with another chemical strike.

But Russia, one of Assad's most fervent allies, dismissed the allegation and suggested that the U.S. was trying to create a pretext for attacking the Syrian government.

Lavrov on Wednesday also cast doubt on Washington's accusations that Assad's government used chemical weapons to attack civilians in April, saying that the U.S. has refused to share its evidence or intelligence assessments.

"We've heard multiple times that the U.S. has a smoking gun, but they cannot share it with us because they cannot compromise their sources and methods," Lavrov said. "Well, I'm not really interested in hearing those statements anymore."

Days after a chemical attack in April, President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE ordered a missile strike on the Shayrat air base in Homs, Syria.

That move drew widespread praise from U.S. allies and fierce condemnation from Russia and the Syrian government, which called it a baseless attack on a sovereign nation.