As tensions rise between Russia and Ukraine, what can President Obama do? The Post's Scott Wilson and Karen DeYoung weigh in. (Jeff Simon/The Washington Post)

Cold War shadows loomed ever larger over the crisis in Ukraine on Monday, as the United States and Russia traded bitter accusations at the United Nations, the Obama administration sought to mobilize international outrage against Moscow, and nervous Eastern Europeans sought Western protection.

With no substantive response to his demands that Russia withdraw its troops from the autonomous Ukrainian region of Crimea, President Obama said Moscow was “on the wrong side of history” and threatened “a whole series of steps — economic, diplomatic — that will isolate Russia and will have a negative impact on Russia’s economy and its status in the world.”

U.S. officials noted with approval the fall of Russian stock values and the ruble when markets opened Monday.

Obama met for more than two hours with his National Security Council on Monday evening to discuss “what steps we can take with our international partners to further isolate Russia and reinforce that the Russians still have an opportunity to take immediate steps to de-escalate the situation or they face further political and economic repercussions,” a senior administration official said.

Separately, the Pentagon announced that it had “put on hold all military-to-military engagements between the United States and Russia,” including “exercises, bilateral meetings, port visits and planning conferences.”

Obama responded sharply to lawmakers who criticized his actions as weak. “I’ve heard a lot of response from Congress about what should be done, what they want to do. One thing they can do right away,” he said, is to join a “unified position that stands outside of partisan politics” to condemn Russian action and approve a package of economic and political assistance for Ukraine’s interim government.

By early evening, Congress appeared on its way to doing so, as senior lawmakers said after a day of meetings in the snow-bound Capitol that they were set to begin debate next week on an economic aid package expected to cost at least $1 billion, along with possible sanctions against senior Russian officials involved in the ongoing military standoff.

Secretary of State John F. Kerry was dispatched to the Ukrainian capital, Kiev, to pledge economic and political support to the interim government, even as he is expected to urge officials there to give diplomacy a chance and refrain from actions that might provoke a shooting war.

In a flurry of meetings from the White House to the United Nations, the European Union and NATO headquarters in Brussels, and the Organization for Security and Cooperation in Europe in Vienna, the United States and its partners threatened Russia with further isolation while offering what they described as a face-
saving solution.

“President Obama has made clear to [Russian] President [Vladimir] Putin that even as we reject and condemn the action that they have taken, that there is a way out of this situation,” Assistant Secretary of State Victoria J. Nuland told OSCE representatives.

The “way out” included an OSCE monitoring mission sent Monday night to Ukraine to provide what Nuland called “neutral facts [and] make a true assessment of the situation on the ground.”

“This monitoring mission can go first and foremost to Crimea to de-escalate tensions and can provide an out for the Russian Federation if it so chooses,” Nuland said. Russia “can pull its forces back” to its existing bases in Ukraine “and have them replaced by independent monitors from the OSCE and from the U.N.”

With the overthrow of President Viktor Yanukovych, all eyes are now on how the Crimeans will react.

Russia has charged that “armed extremists” have taken over in Ukraine and pose a danger to the majority-Russian population in Crimea and other pro-Russian areas in the eastern part of the country, allegations that the United States and others deemed false. Although Russia is an OSCE member, it has not agreed to allow monitors to travel to Crimea, where Russian troops are in control of all border posts, military bases and regional government facilities.

Although there has been no suggestion of Western military intervention, some of NATO’s Eastern European members have made clear that they take seriously the mutual-defense provisions of the alliance charter.

A meeting of NATO’s North Atlantic Council was scheduled for Tuesday after Poland said it felt threatened by Moscow’s moves in the region. The meeting came under the same provision that led the alliance early last year to deploy Patriot missile batteries in response to a Turkish request for protection along the border with conflict-ridden Syria.

The Polish appeal for NATO “consultations” did not spell out what action was being sought, but Poland’s embassy in Washington said in a statement that “we decided we could not wait any longer.”

“Russian military activity is not limited to blatant provocations and unlawful intervention in Crimea, or the threat of massive invasion on the territory of sovereign Ukraine,” the statement said. It noted that 150,000 Russian troops and battle tanks recently participated in military drills near the Polish border, and that another was initiated Monday — with Putin in attendance — in the nearby Kaliningrad region.

That was “more than enough reason” for Polish concerns, the statement said. The NATO meeting, it said, would review “the measures to be taken to safeguard the security interests of the Allies.”

Not every NATO member seemed entirely on board with the escalating rhetoric. German officials, in particular, said they were concerned about the rising volume of charges and countercharges. By keeping its rhetoric powder dry, said officials who spoke on the condition of anonymity about internal deliberations, Germany hoped to keep clearer diplomatic channels to Moscow open.

Meanwhile, at a U.N. Security Council session reminiscent of debates over Soviet expansion in the 1960s, the United States, Britain and France accused Russia of fabricating its justifications for military intervention.

Russia’s ambassador to the United Nations, Vitaly Churkin, charged that Moscow had sent troops to head off an “armed takeover by radical extremists” chanting “anti-Russian, anti-Semetic slogans,” leading U.S. Ambassador Samantha Power to counter that such allegations were “without basis in reality.”

“There is no evidence of violence against Russian or pro-Russian communities” in Crimea, Power said.

Churkin’s insistence that Russia had not sent any troops to Crimea beyond those already based there as part of its Black Sea Fleet and other long-standing deployments drew sharp expressions of disbelief from other diplomats in an hours-long session that descended into increasingly sarcastic comebacks.

“Listening to the representative of Russia,” Power said, “one might think that Moscow had just become the rapid-response arm of the [U.N.] High Commissioner for Refugees.”

French Ambassador Gérard Araud went one step further, saying that he was “hearing the voice of the past. I was 15 years old in 1968 when Soviet forces entered Czechoslovakia. . . . [with] the same justification. . . . It is propaganda that denies reality.”

Jen Psaki, a spokeswoman for the State Department, said the administration is “highly likely” to impose additional economic and political sanctions on Russia unless it sends its troops in Crimea back to Russian bases and engages in dialogue with the Kiev government.

“We are not just considering” additional steps, Psaki said. “It is likely we will put [sanctions] in place. . . . We are preparing options, and we are likely moving down that path.”

“We are far more forward on this than we were even yesterday,” she said.

Rear Adm. John F. Kirby, a spokesman for the Pentagon, said the ban on contacts with the Russian military included events such as Northern Eagle, a series of joint anti-terrorism and anti-piracy exercises, scheduled for May, among the United States, Norway and Russia, and the U.S.-Russia Defense Relations Working Group, established in 2010 to share best practices related to military training and education and to discuss “issues of common security interest,” such as the Middle East and Afghanistan.

In a briefing for reporters late Sunday, senior State Department officials spoke of “a broad menu of options to curtail our economic and trade relationship, to look at pressure on individuals who may have been responsible, and to curtail normal activity that we have ongoing with Russia at this time.”

On Capitol Hill, some lawmakers said they did not think the administration was moving far enough, fast enough.

“We’re trying to lead on this, because our concern is that the administration, in only using rhetoric and not taking a decisive step . . . is allowing Putin to believe that he can move forward without any downside risk,” said Rep. Edward R. Royce (R-Calif.), chairman of the House Foreign Affairs Committee.

Ed O’Keefe contributed to this report.