WASHINGTON (AP) — Western powers on Sunday prepared a tough response to Russia’s military advance into Ukraine and warned that Moscow could face economic penalties, diplomatic isolation and bolstered allied defenses in Europe unless it retreats.

The crisis may prove to be a game-changer for President Barack Obama’s national security policy, forcing him to give up his foreign policy shift to Asia and to maintain U.S. troop levels in Europe to limit Russia’s reach.

The ill will and mistrust also could spill over on two other global security fronts — Syria and Iran — where Russia has been a necessary partner with the West.

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.”

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. The officials were not authorized to publicly discuss the situation and spoke on condition of anonymity.

Secretary of State John Kerry said he has consulted with other world leaders, and “every single one of them are prepared to go to the hilt in order to isolate Russia with respect to this invasion.” 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. Officials said the Obama administration would also focus this week on putting together a package of economic assistance for Ukraine.

The White House issued a joint statement Sunday evening on behalf of the Group of Seven saying the G-7 had suspended participation in the planning for an international summit in Russia this summer. In the statement, the U.S., Canada, France, Germany, Italy, Japan and the United Kingdom condemned Russia’s “clear violation of the sovereignty and territorial integrity of Ukraine.” They said Russia’s advances in Ukraine violate the “principles and values” on which the G-7 and Group of Eight economic group, which includes Russia, operate.

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.

“There are very serious repercussions that can flow out of this,” Kerry said.

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.

Treasury Secretary Jacob Lew told a Washington audience on Sunday that the United States is ready to work with other countries and the International Monetary Fund to provide support to bolster Ukraine’s economy. He said he had been assured in discussions with Ukrainian officials that the new government is prepared to pursue the necessary reforms to overhaul the country’s ailing economy.

Lew said that the administration was ready to supplement emergency IMF loans to cushion the impact economic reforms would have on vulnerable Ukrainians.

“The United States is prepared to work with its bilateral and multilateral partners to provide as much support asUkraine needs to restore financial stability and return to economic growth if the new government implements the necessary reforms, » Lew said during a speech Sunday night to the annual policy conference of AIPAC, America›s largest pro-Israel lobbying group.

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. “The people of Ukraine will not sit still for this. They know how to fight.”

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 in Ukraine.

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.” One of the administration officials indicated that the U.S. was not weighing military action to counter Russia’s advances, saying the Obama administration’s efforts were focused on political, economic and diplomatic options.

Rubio said it would be difficult to rein in Moscow. He said Putin has “made a cost-benefit analysis. He has weighed the costs of doing what he’s done, and … clearly he has concluded that the benefits far outweigh the costs. We need to endeavor to change that calculus.”

As a starter, Rubio and fellow GOP Sen. Lindsey Graham of South Carolina said the Obama administration should return to plans it abandoned in 2009 to place long-range missile interceptors and radar in Poland and the Czech Republic.

Russia believed the program was aimed at countering its own missiles and undermining its nuclear deterrent. The White House denied that and has worked instead to place medium-range interceptors in Poland and Romania — aimed at stopping missiles from Iran and North Korea.

Experts said potential U.S. budget cuts to Army units based in Germany also could be slowed, or scrapped completely, to prevent a catastrophic erosion of stability and democracy from creeping across Europe.

The Pentagon is considering new reductions to Army units in Germany that already have been slashed under Obama. Currently, there are two Army brigades — up to 10,000 soldiers — based in Germany, where armored and infantry units have dug in since World War II. At the end of the Cold War, more than 200,000 American forces were stationed across Europe.

Damon Wilson, an Eastern European scholar, former diplomat and executive vice president of the Washington-based Atlantic Council think tank, said the U.S. must be ready to pour its efforts into Ukraine, even at the cost of policies and priorities elsewhere.

“We should be no longer deluded by the fact that Europe is a safe spot of stability and security, and not a security risk for the U.S.,” Wilson said Sunday. He said that if Putin goes unchecked, it could result in war — the second one on NATO’s borders.

The 3-year-old civil war in Syria is already a crisis for neighboring Turkey, a NATO member state. Ukraine is not a NATO member, but it borders four nations that are — Poland, Slovakia, Hungary and Romania.

“This is the biggest challenge to Obama’s presidency,” Wilson said. “This is a pretty tectonic shift in our perception of European security.”

Wilson said the White House may have to abandon the policy shift to Asia — its attempt to boost America’s military, diplomatic and economic presence there — to refocus on Russia’s threat.

He played down concerns that the new schism between Washington and Moscow will have an effect on efforts to end the war in Syria and limit Iran’s nuclear program.

In Syria, Wilson said, Russia relied on a “bankrupt plan” in its failure to convince President Bashar Assad to embrace peace. “There’s nothing happening there that’s credible in a positive way,” he said.

With Iran, the bulk of negotiations already have been between the U.S. and Iran, said Wilson, who described Russia as mostly playing in the background.

Even so, officials said the U.S. and the West would not be able to roll over Russia on any number of global diplomatic or economic fronts.

Russia has made clear it is ready to provide weapons and military equipment to governments across the Mideast that have irked Washington. Russia’s permanent seat on the U.N. Security Council gives it veto power over major world deliberations.

“The challenge is, we do need to have some kind of working relationship with Russia,” Rep. Adam Schiff, D-Calif., said Sunday. “And while we can impose these costs and take these steps, we’ve got to be mindful of the fact that they can impose their own costs on us.”

Kerry appeared on CBS’ “Face the Nation,” ABC’s “This Week” and NBC’s “Meet the Press.” Rubio was NBC, while Graham and Schiff were interviewed on CNN.

Lara Jakes, AP National Security Writer.