"I think we need to be as open as we possibly can to solutions now," Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff, told a breakfast with reporters.
Sens. Carl Levin, John McCain and Jim Webb last month called for the United States to rethink base realignment plans in East Asia due to political opposition in Japan and cost overruns in both Japan and South Korea.
Japanese and U.S. officials earlier dismissed the proposal, saying that the plan worked out in 2006 was the best approach and would reduce troop numbers on Okinawa Island, where tensions with local residents have been frequent.
Mullen, whose term ends in September, did not comment specifically on the senators' proposal but said that the United States needed to be mindful both of costs and the political situation in Japan.
"This thing's been discussed for 15 years. There have been 20 different kinds of solutions that I've seen - all of them are difficult. So I think we have to be realistic here," Mullen said.
Any solution needs to preserve "the kind of influence and stability that our presence in that part of the world has done for 60-plus years, and at the same time recognize limitations and that there are needs on both sides."
The senators said that Japan needed to focus on reconstruction from its massive March 11 earthquake and not be distracted by the base dispute, which contributed to the resignation of a prime minister, Yukio Hatoyama, last year.
Under the 2006 plan, the United States would move the flash point Futenma base out of a crowded urban area and to an isolated stretch of beach elsewhere on Okinawa.
Some Okinawan activists have demanded that the base be removed from the island completely. The senators said Futenma was needed for security but that its functions could largely be shifted to Okinawa's existing Kadena Air Base.
The United States stations 47,000 troops in Japan under a post-World War II security treaty, with half in Okinawa. The 2006 plan aims to shift 8,000 Marines and their families from Okinawa to the U.S. territory of Guam in 2014.