MUNICH, Germany - As the U.S. unfolds plans to withdraw troops from Europe in a strategic shift toward Asia and the Middle East, top U.S. officials here sought to reassure European allies that working together is key to facing global challenges.

Missile defense and tensions with Iran are among important areas of cooperation between the U.S. and Europe, Secretary of Defense Leon Panetta and Secretary of State Hillary Rodham Clinton said in consecutive speeches at the annual Munich Security Conference on Saturday.

"When Americans envision the future, we see Europeans as our essential partners," Clinton said. "There is no greater sign of our confidence and commitment than just how much we hope and need to accomplish with you."

Panetta called the proposed withdrawal of U.S. troops from Europe a "vote of confidence" in the continent by the Obama administration.

Discussion at the annual conference, a forum for leaders of government and industry to discuss a broad range of issues affecting security, also turned to the economic challenges faced by both Europe and the United States and the resulting austerity measures affecting defense spending.

As the U.S. scales back in Europe as part of the new defense strategy and pending budget cuts, Panetta urged European nations to increase their commitment to NATO.

"We must all continue to invest in national defense and in the shared responsibility and capabilities of NATO in order to best manage the security challenges of the future," Panetta said.

Speaking to a European audience for the first time since the release of the Defense Department's new strategic guidance, Panetta shared details of the coming U.S. military shift, marked by the withdrawal of two heavy Army brigades from Europe.

The U.S. soon will name a U.S.-based brigade to contribute to the NATO Response Force, a 25,000-strong multinational force aimed at quick response to international emergencies. Crafted in 2003, the force has seen little involvement to this point from the U.S. due to the wars in Iraq and Afghanistan, Panetta said.

"Although it will evolve in light of the strategic guidance and the resulting budget decisions, our military footprint in Europe will remain larger than in any other region in the world," he concluded.

The defense secretary also stressed that NATO allies agree that international forces in Afghanistan would remain combat capable through the end of 2014, although they hope Afghan forces will assume the lead combat role by 2013. Comments by Panetta last week suggesting the U.S. could end its combat role in 2013 to focus on supporting Afghan forces had generated some controversy.

"Of course [NATO forces] will remain fully combat capable," he said. "And we will engage in combat alongside Afghan forces as necessary."

Panetta said a European missile defense system, controlled from Germany, will provide a response to the "most pressing" security challenges emerging outside the continent. U.S. plans call for stationing missiles in Poland and Romania, a radar station in Turkey and four Aegis-equipped ships to be based in Rota, Spain.

Russia opposes a missile shield as planned, and its foreign minister again on Saturday expressed concerns about the system. Sergei Lavrov said Russian offers to work together with NATO and the U.S. have been ignored.

In comments on Saturday, German Foreign Minister Guido Westerwelle said any missile defense plan must include Russia. He also questioned the need for increased defense budget spending.

"In times of austerity, it cannot be about spending more money on defense," Westerwelle said. "We need to be more sensible and smarter about making use of our resources." That sentiment was shared by British Defence Secretary Phillip Hammond, who said the military establishment needed to take a lesson from industry, challenging "the notion that if you put less in, you have to get less out."

However, U.S. Sen. Lindsey Graham, R-S.C., a member of the Senate Armed Services Committee, warned that Congress' unquestioned support for NATO is no longer guaranteed, given the mood of the nation at a time of high debt.

"If I introduced legislation next week saying that if NATO doesn't maintain 2 percent of GDP spending on defense we would withdraw, it actually would pass," Graham said.

As the conference turned to Asia, Sen. John McCain, ranking member of the Armed Services Committee, again questioned U.S. base plans in Okinawa, Guam and Korea, calling them "grossly overbudgeted." The Republican from Arizona lead Senate efforts last year to freeze funding for a planned shift of 8,600 Marines from Okinawa to Guam, a move expected to cost the U.S. roughly $22 billion. Senators instead wanted more joint basing agreements with Pacific countries, similar to the recent U.S. agreement to rotate Marines through Darwin, Australia.

McCain did not offer a reaction to recent reports that the Obama administration is considering sending 4,500 Marines to Guam and rotating others through joint bases in the Philippines and Australia.

He did criticize the Defense Department's decisions to slow the pace of combat ship building and retire seven cruisers early. The cuts would be "catastrophic," he said, and would keep the Navy shy of its long-standing goal of 313 combat ships.

"If the U.S. military is to maintain a stabilizing presence in the Pacific, we must maintain the means of being present," McCain said.

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