WASHINGTON — In a sudden reprise of Cold War sensibilities, the U.S. and its allies are weighing sanctions on Moscow and whether to bolster defenses in Europe in response to Russia’s military advances on Ukraine.

Secretary of State John Kerry, soon on his way to Ukraine’s capital, said world leaders “are prepared to go to the hilt in order to isolate Russia with respect to this invasion.”

Much as when superpower tensions ruled world affairs, missile defense systems and troop levels in Europe have again become urgent questions in Washington and beyond, a renewed reality that may force President Barack Obama’s administration to give up its intended foreign policy shift to Asia indefinitely.

Also echoing the era of East-West confrontation, there appears to be little if any taste in the West for a direct military response to Russia’s provocation.

Russian President Vladimir Putin gave no indication that he would heed the West’s warnings. Hundreds of armed men surrounded a Ukrainian military base in Crimea, a pro-Russian area. In Kiev, Ukraine’s capital, Prime Minister Arseniy Yatsenyuk alerted allies that “we are on the brink of disaster.”

“This is absolutely the most serious test of our alliances since the Cold War ended,” Rep. Marcy Kaptur, D-Ohio, said in a nationally broadcast interview today.

“I think it is extremely dangerous. Ukrainians fight and Russians fight,” said Kaptur, who has traveled to Ukraine on several occasions and is considered an expert on that part of the world.

Senior Obama administration officials said they believe Russia now has complete operational control over Crimea and has more than 6,000 forces in the region. The U.S. was also watching for ethnic skirmishes in other areas of eastern Ukraine, though the officials said they had not yet seen Russian military moves elsewhere.

Kerry said he has consulted other world leaders and all are committed to doing what is necessary to isolate Russia diplomatically. President Barack Obama spoke Sunday with German Chancellor Angela Merkel, British Prime Minister David Cameron and Polish President Bronislaw Komorowski.

Kerry planned to travel to Kiev Tuesday for meetings with the Ukrainian government. Treasury Secretary Jacob Lew said the U.S. is ready to work with other countries and the International Monetary Fund to provide support for Ukraine’s economy. In Brussels, NATO’s secretary-general, Anders Fogh Rasmussen, said Russia’s actions have violated a U.N. charter. He said the alliance was re-evaluating its relationship with Russia.

Beyond economic sanctions and visa bans, freezing Russian assets, and trade and investment penalties, Kerry said Moscow risks being booted out of the powerful Group of Eight group of world powers as payback for the military incursion.

Former Ambassador Nicholas Burns said, “Putin’s not going to back off. ... What can President Obama do? Be very minded in opposition. We can’t follow a military policy. This has to be diplomatic.”

Several U.S. senators also called for bolstered missile defense systems based in Poland and the Czech Republic.

Russia is “going to be inviting major difficulties for the long term,” said Kerry.

Still, it was clear that few in the West were prepared to respond immediately to Putin with military force.

At the Vatican, Pope Francis used his traditional Sunday midday appearance in St. Peter’s Square to urge world leaders to promote dialogue as a way of resolving the crisis.

Sen. Marco Rubio, R-Fla., discussing the potential of U.S. military strikes against Russian troops in Crimea, said, “I don’t think anyone is advocating for that.” An administration official indicated that the U.S. was not weighing military action, saying that efforts were focused on political, economic and diplomatic options.