Russian warplanes struck rebels in Syria’s last major opposition stronghold on Tuesday, forcing the Trump administration to confront a looming regime offensive that is expected to deliver a fatal blow in the seven-year conflict and hand Moscow a diplomatic victory.

The Trump administration issued a coordinated call for Russia, Iran and Syria to avoid a military assault on Idlib. President Trump, Secretary of State Mike Pompeo, United Nations Ambassador Nikki Haley, and Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, all warned of a potential catastrophe if the battle goes forward.

Moscow has dismissed U.S. concerns that Syria again will use chemical weapons during the offensive, a move that has twice led Mr. Trump to order limited airstrikes targeting the Syrian regime. The Kremlin’s response has fueled widespread skepticism in Washington about Moscow’s intentions, and raised fears that Russia will give Syria a free hand to use chemical weapons on civilians as Syrian forces prepare for a battle that could deliver a crippling blow to rebels who have been fighting the regime.

“We will find out and learn the extent to which the U.S. has power short of deploying military force,” said Nicholas Heras, a Syria expert at the Center for a New American Security. “The Trump administration has one more chance to carve a line in the sand.”

U.S. officials have outlined no plans to intervene militarily in an assault, but the Trump administration has warned that it could again hit the Syrian regime if it uses chemical weapons.