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.