BRUSSELS — Already concerned by the Trump administration's erratic approach to the trans-Atlantic alliance, European security services have been shaken by the resignation of US national security adviser Michael Flynn over allegations he lied about his contacts with Russian diplomats, amid increasing concern that the new US administration is pursuing an uncomfortably close relationship with NATO's foremost rival, Russia.

Flynn’s resignation Monday night immediately sent European officials into a frenzy of attempting to determine what the change of the president’s top national security adviser would mean as the Atlantic alliance has already been struggling with understanding how the new president will approach a litany of complex European situations from the expansion of NATO to the war against ISIS to concerns about an expansionist Russia.

"I was hoping you could tell me what the fuck is going on over there,” said one European Union intelligence official who, like the other officials contacted, declined to speak about such a diplomatically sensitive situation on the record.

"There's no guide for handling this sort of situation, happening with such an important and powerful ally,” the official said. “If anything, it's a wake-up call to European leaders that counting on America isn't currently a smart policy. Of course this is exactly what Putin wants — to destabilize the Atlantic alliance — but I have to counsel my policymakers the best I can, and right now it's ‘Prepare to handle some crises without US support.’"

The perception that Flynn had close ties to Russian intelligence and diplomats had alarmed a number of observers from Washington to Brussels, as had his reputation as a poor manager after his 2014 firing as the head of the Defense Intelligence Agency by then-President Barack Obama. The concerns about his ties to Russia were inflamed by the revelations that, during the transition last winter, he had possibly told Russian officials the incoming administration was open to reconsidering the sanctions Obama had just imposed, and then denied having done so.

While some observers and allies were reassured by Flynn’s departure, most stated that the broader problem with the Trump administration is an opaque and conflicting policymaking process that had left allies uncertain on how key Trump officials see the NATO alliance and relations with the European Union.

"This changes nothing in terms of European concerns about possible shifts in American foreign policy from this administration when it comes to the role of NATO and the EU,” said a NATO official, who declined to be identified discussing high-level diplomacy.