The Trump administration is plotting to inflict “total economic isolation” on Russia if it backs the Assad regime’s assault of Syria’s opposition stronghold of Idlib, and is working with European allies to send that warning, a U.S. official told The Daily Beast.

“This doesn’t just apply if they use chemical weapons,” the U.S. official said, speaking anonymously because he was not authorized to speak publicly. “This is if they help assault the city.”

The warning comes as residents say the Syrian government continued air strikes over the weekend with Russian backing against targets in Idlib province. An estimated 30,000 opposition fighters are sheltering in Idlib, one of the last rebel strongholds in Syria—together with nearly 3 million people including a million children, according to UN estimates. The UN is warning that at least 700,000 people could be displaced by the offensive.

The administration’s top State Department officials on Syria— Special Representative for Syria Engagement Amb. Jim Jeffery and Special Envoy Joel Rayburn—headed to Geneva Monday to build a European consensus on the firm warning to Moscow and to Syria, a State Department official told The Daily Beast, speaking anonymously as a condition of discussing the meetings. “ They will engage with like-minded countries regarding the situation in Idlib, reaffirm our position that any military offensive would be a reckless escalation of the conflict, and push for progress in the Geneva process,” the official said.

Jeffery told Reuters and a handful of reporters last week that a “major diplomatic initiative” was needed to resolve the 7-year conflict.

President Donald Trump has tweeted his disapproval of any invasion of Idlib, and the White House released a statement that the U.S. and its allies “will respond swiftly and appropriately... if President Bashar al-Assad chooses to again use chemical weapons.”

European countries are already sending their own warnings against an assault, with Germany announcing Monday that it would consider deploying its own military to Syria if chemical weapons are used.

“We are in talks with our American and European partners about this situation,” German government spokesman Steffen Seibert said Monday, according to Reuters.

The U.N. Security Council warned Friday against an assault, but gave no teeth to that warning in the form of threatened punitive action. The presidents of Russia, Turkey and Iran also failed to reach agreement at a summit in Tehran on Friday to head off an attack, with Iran calling for a full-court press to drive both Syrian opposition and U.S. forces out of the country, and Turkey arguing for a ceasefire, especially as it has troops inside Idlib.

Putin called a ceasefire “pointless” after the summit, and he’d earlier warned that rebel fighters were making preparations to attack Assad’s forces, including preparing chemical weapons—a possible attempt to deflect suspicions of who carried out any such attack.

Jeffery said last week that the U.S. had determined that it was Assad preparing to use such weapons, and The Wall Street Journal reported Sunday that Assad had approved a chlorine gas attack on Idlib.

The spokesman for the Russian embassy in Washington, Nikolay Lakhonin, said he had “no reaction... at the moment” to the plans to punish Rusdia for an Idlib offensive. But as a demonstration of the complex task U.S. officials face in any effort to isolate Moscow, he sent a link to a Kremlin statement on Russian President Vladimir Putin’s meeting with Japan’s Prime Minister Shinzo Abe on Monday to discuss “economic, humanitarian and military cooperation.”

The statement said a “package of bilateral documents was signed, including documents on satellite communications, joint investment, financial cooperation, construction of a polymer plant, and implementation of a project to set up a chemical cluster in Russia.”

A senior Western official in Washington, D.C., predicted the assault on Idlib would be one area that would find widespread agreement, even from European officials previously leery of stepping up sanctions.

“There’s a desire to put pressure on the Kremlin… We stand with our US partners in terms of the actions we take to deter and rein Russia in," a second senior Western official said, though the official was not aware of a specific new effort ahead of a possible Syrian-Russian assault on Idlib. Both officials spoke anonymously because they were not authorized to discuss the matter publicly.

"The work now is how do we structure and build on the sanctions regime we have in the EU,” the European Union, the second official said. “The U.S. system has a second round of sanctions in 90 days. With EU partners, we are looking at what more we can do, particularly around a chemical weapons regime. We're also looking at what measures will have the most impact—how do you seek to change the behavior and the Kremlin’s calculations?"

“I welcome the notion that this administration is prepared to do this… but it comes with some real risk,” said Jonathan Schanzer, a former Treasury expert on sanctions and terror finance, now senior vice president at think tank FDD. “The Russians play dirty and they play rough. You have to think about not just offensive economic warfare but also the defensive practices that are necessary to protect allies” from economic payback—like cutting off Russian gas supplies to European countries ahead of winter.

Schanzer said the U.S. has a menu of options available, the most dangerous in terms of blowback being sanctions against Russian industrial sectors like the defense companies supplying the Russian war machine. “That’s why sanctions thus far have been calculated and surgical. One has to be careful when escalating in this way. Economic warfare can lead to warfare,” he said.