“This is a wake-up call, for the Euro-Atlantic community, for NATO and for all those committed to a Europe whole, free and at peace,” he said. “We had thought that such behavior had been confined to history, but it’s back, and it’s dangerous because it violates international norms of accepted behavior.”

Mr. Rasmussen said the alliance was reviewing the full range of its cooperation with Moscow and had suspended its plans to escort Russian ships that are ferrying chemicals for making poison gas from Syria. The alliance has also canceled staff-level meetings between NATO and Russian officials, though it has kept the door open to political talks.

NATO members have also taken a series of relatively modest military steps to reassure its East European members. The United States has sent six F-15 fighters to Lithuania to bolster NATO’s air policing mission in the Baltic states and has sent 12 F-16s to Poland, which borders Ukraine.

Two NATO surveillance planes are patrolling Polish and Romanian airspace. Britain also recently announced that it planned to send several Typhoon aircraft to join the Baltic mission.

Mr. Rasmussen said that he expected additional steps, but he did not say what they might be.

Some experts say the Western alliance should reconsider the assurance it provided Russia in 1997 that NATO would not deploy a substantial number of ground forces on the territory of its Central and East European members. That assurance was part of an agreement on cooperation between NATO and Russia.

“We ought to take another look at having a visible forward presence on the ground and in the air in Central and Eastern Europe,” said Ivo H. Daalder, president of the Chicago Council on Global Affairs, who recently served as the United States ambassador to NATO.

Other experts have also focused on that possibility.

“To build further confidence in NATO’s collective-security commitments to members in its eastern area, Washington should return to Europe a third brigade combat team,” William Courtney, the former American ambassador to Georgia and Kazakhstan, and Job C. Henning, a fellow at the Center for the Study of the Presidency and Congress, wrote in The National Interest. “If requested by Poland, it ought to be based there.”

The Obama administration has not signaled what additional steps it is prepared to take, but noted that it would participate in a previously planned multinational military exercise in Ukraine this summer, called Rapid Trident. Mr. Hagel spoke by phone with his Ukrainian counterpart, Ihor Tenyukh, on Wednesday. Carlos Pascual, the State Department’s special envoy for international energy affairs, left on Wednesday for a meeting in Kiev on how to lessen Ukraine’s energy dependence on Russia.