The U.S. and Japanese governments said Thursday that they will move about 9,000 Marines off Okinawa to other bases in the Western Pacific, seeking to resolve longstanding tensions over the facility. (Issei Kato/Reuters)

The U.S. and Japanese governments said Thursday that they will move about 9,000 Marines off Okinawa to other bases in the Western Pacific, in a bid to remove a persistent irritant in the relationship between the two allies.

The Marine Corps Air Station Futenma on Okinawa has been seen by both sides as essential to deterring Chinese military aggression in the region. But the noisy air base’s location in a crowded urban area has long angered Okinawa residents, and some viewed the Marines as rowdy and potentially violent.

“I am very pleased that, after many years, we have reached this important agreement and plan of action,” Defense Secretary Leon E. Panetta said in a statement.

Still unresolved is the issue of establishing a replacement for Futenma. The failure to find a suitable spot for a new air base had held up a previous effort to relocate the Marines to Guam, but the current agreement removes that barrier. U.S. Marines would leave Futenma as soon as suitable facilities on Guam and elsewhere are ready.

The earlier plan in 2006 to relocate the base had been plagued by financial and political difficulties in both the United States and Japan. Frustration over the failure to execute that agreement grew so intense that it contributed to the resignation of Prime Minister Yukio Hatoyama in 2010.

Under the current plan, the total cost of closing Futenma and transferring the 9,000 Marines off Okinawa will be about $8.6 billion. The Japanese government will pay about $3.1 billion to facilitate the moves.

About 5,000 of the Marines will go to Guam, and a smaller number will head to other locations in the Pacific, such as Hawaii or Australia. Even after the moves, about 10,000 Marines will remain on Okinawa, as called for under the earlier agreement.

Japanese officials on Friday offered mixed messages about their commitment to relocate the Futenma base to a less populated strip in Okinawa. Defense Minister Naoki Tanaka said during a press conference that the 2006 plan was still the only valid solution. But Foreign Minister Koichiro Gemba said, according to the Kyodo news agency, that other relocation options for the Futenma base might be considered.

The plan announced Thursday appears to have somewhat placated three senior U.S. senators on the Armed Services Committee, who this week raised concerns about costs and about how the move would affect broader military strategy in the region.

In a statement, Armed Services Chairman Carl Levin (D-Mich.), ranking Republican John McCain (Ariz.) and James Webb (D-Va.) said the revised plan had addressed “some” of the issues they raised.

“We still have many questions about the specific details of this statement and its implications for our force posture in the Asia-Pacific region, and we will continue to work with the Administration and the Government of Japan to achieve the objectives we all share,” the three senators said in a statement.

Earlier this week, they wrote a letter to Panetta raising doubts about the emerging proposal. They questioned “cost estimates, military sustainment and force management, and how it would support a broader strategic concept of operations in this increasingly vital region.”

The senators suggested that no plan should be considered final without the support of Congress, which controls spending on base construction.

U.S. officials said the failure to come to an agreement on the closure of the Futenma air base was hindering the overall American-Japanese alliance.

“Because we’ve been spending so much time talking about the move from Futenma, we’re not making as much progress as we would have liked in other aspects of the alliance,” said a senior State Department official. The agreement should make it easier for the United States and Japan to advance on other issues, such as cybersecurity, intelligence sharing and missile defense, the official said.

The U.S.-Japanese alliance is seen as essential to deterring Chinese efforts to dominate the region and reinforcing U.S. and South Korean troops in the event of a war with North Korea.

A joint Japanese-American statement issued Thursday night said that the “increasingly uncertain security environment” in the Asia-Pacific region required a robust U.S. military presence.

U.S. officials said that moving the Marines off Okinawa to several bases in the Western Pacific would give the Americans a force in the region that is more capable and less vulnerable to attack because it is more geographically distributed.

Correspondent Chico Harlan in Tokyo contributed to this report.