MUNICH — Millions of Syrian refugees could soon flee for Europe to escape Syrian President Bashar Assad’s Russian-backed assault on the last major rebel holdout in the country, according to U.S. officials.

“The offensive in Idlib, which, if it is not stopped, will create a true humanitarian catastrophe of 3 million people pouring across the Turkish border,” a senior administration official told reporters on the sidelines of the Munich Security Conference. “They’ll be spewed onto Europe.”

That warning portends the intensification of a crisis that has reverberated around the world, stoking fears of terrorism emanating from the war-torn nation and upending political careers as governments struggled to address the challenge. The pressure for refugees to flee is building as Western allies assembled in Germany for the world’s premier foreign policy conference, where the unease about the crisis hasn’t translated into any specific plans to address it.

“That always hangs over as one of the great concerns ... and why we focus on and why our European partners are interested in Syria, in Libya for that matter,” another senior administration official said of the refugee issue on Saturday. “It hasn’t come up in terms of actual logistics, dynamics, planning.”

The Syrian refugee crisis has caused tensions between NATO allies and within Western democracies. Outgoing German Chancellor Angela Merkel’s openhanded policy towards the refugees did mortal damage to her political career and boosted the campaign to lead the United Kingdom out of the European Union.

Turkish President Recep Tayyip Erdogan has threatened to “open the gates” and send refugees onto Western Europe during recent controversies, but the impending crisis isn’t a manufactured one, U.S. officials concede.

“They already have 3.5 million; they can’t absorb anything like that,” the first senior administration official, speaking Friday evening, said of Turkey. “We’ve seen almost a million people — over 800,000 — up and move in the last few days. And this is the largest single mass movement we’ve seen in the entire Syrian war, which is saying a lot considering that you’ve gotten almost 11 million people have at one or another point left their homes.”

Secretary of State Mike Pompeo’s team has blamed Moscow for the brewing problem, because the Russian military is supporting Assad’s latest military offensive rather than bringing the regime to negotiate a political settlement.

“Russia apparently feels that it cannot bring the regime to do the necessary things to bring it in line with the international community's expectations and needs, so the Russians are going to press forward for a military victory,” James Jeffrey, the State Department’s lead negotiator for the Syria crisis, surmised recently.

That decision now threatens to swamp the capacities of humanitarian organizations and the Turkish government.

“We’re working with humanitarian agencies and the U.N. and all those U.N.-associated agencies to ensure that there’ll be immediate humanitarian aid, but we can’t handle or absorb a million, let alone 3 million people, without considerable dislocation,” the first senior administration official said. "The U.N. agencies, I think, can handle many hundreds of thousands, but boy, even in the short run, this will be really, really tough. We’re really worried.”