Russian President Vladimir Putin is preparing to launch unilateral air strikes against Isil from inside Syria if the US rejects his proposal to join forces.
Mr Putin's preferred course of action, though, is for America and its allies to agree to coordinate their campaign against the terrorist group with Russia, Iran and the Syrian army, which the Obama administration has so far resisted, according to a person close to the Kremlin and an adviser to the Defence Ministry in Moscow.
Russian diplomacy has shifted into overdrive as Mr Putin seeks to avoid the collapse of the embattled regime of Bashar al-Assad, a long-time ally who is fighting both a civil war and Sunni extremists under the banner of Isil.
Israeli Prime Minister Benjamin Netanyahu flew to Moscow for talks with Mr Putin this week, followed by Turkish President Recep Tayyip Erdogan.
Mr Putin's proposal, which Russia has already communicated to the US, calls for a "parallel track" of joint military action accompanied by a political transition away from Assad, a key US demand, according to sources.
The initiative will be the centrepiece of Putin's one-day trip to New York for the United Nations General Assembly on September 28, which may include talks with President Barack Obama.
"Russia is hoping common sense will prevail and Obama takes Putin's outstretched hand," said Elena Suponina, a senior Middle East analyst at the Institute of Strategic Studies, which advises the Kremlin. "But Putin will act anyway if this doesn't happen."
Putin's military build-up in Syria in recent weeks has alarmed US officials who are still outraged by his annexation of Crimea and support for the insurgency in Ukraine, which prompted the American and European sanctions that have helped push Russia's economy into recession.
The US is willing to discuss coordinating strikes to avoid hostile incidents with Russian planes, but America and its allies haven't received a "concrete" proposal from Moscow and won't include Assad's forces in the effort, an official in Washington said, speaking on condition of anonymity.
"I think what you'll see between the United States and Russia is tactical deconfliction so we can ensure safe operations for our service members," US ambassador to the United Nations Samantha Power told Bloomberg.
"Ultimately, as long Assad is governing, given his record and his ongoing actions, we're just not going to be able to have the kind of success that we need in defeating Isis," she said, using another acronym for Isil.