YOKOHAMA, Japan » The flare-up of territorial disputes souring relations between Japan, China and Russia has not weakened the U.S.-Japanese security alliance and has underscored the importance of America’s regional military presence, Japan’s leader said Saturday.

Prime Minister Naoto Kan, after meeting President Barack Obama on the sidelines of the Asia-Pacific Economic Cooperation economic summit, said he thanked the U.S. for its support during the disputes, which he said demonstrated the importance of the U.S.-Japan security alliance and the presence of U.S. troops.

Japan’s relations with Beijing remain chilly after a Chinese fishing trawler collided with Japanese patrol boats in September off disputed islands to its south. Not long after that, Russian leader Dmitry Medvedev paid a surprise visit to another disputed island off Japan’s northern shores, a move that prompted a strong protest from Tokyo.

"We have had various problems in our relations with Russia and China, and I thank President Obama for his support," Kan said. "Not only the Japanese people, but also our neighboring countries have realized the importance of the United States and the presence of the U.S. military."

Washington, which has about 50,000 troops in Japan under a decades-old mutual security treaty that binds it to protect Japan, has offered to mediate talks between Tokyo and Beijing to help resolve the dispute with China. Beijing, however, has refused to participate in any such forum.

Obama stressed Washington’s continued support of the alliance, which marked its 50th anniversary this year, and its value as a stabilizing factor in the region. Officials on both sides of the Pacific have long hailed the alliance as a strong deterrent in Asia, even if troops are not directly called on to intervene in disputes.

"The commitment of the United States to the defense of Japan is unshakable," he said. "Japan and the United States are stronger when we stand together."

The leaders largely sidestepped issues within the alliance itself.

The two countries have been moving to strengthen their military cooperation — which both see as an important factor in maintaining security in the region as China’s military grows in strength. But the alliance itself has been strained by an ongoing disagreement over what to do about a major U.S. Marine Corps facility on the southern island of Okinawa.

Tokyo and Washington agreed years ago to move the base, called the Marine Corps Air Station Futenma, to a less crowded part of the island, and Washington is planning to relocate more than 8,000 Marines off Okinawa altogether to the U.S. Pacific territory of Guam by 2014.

But Washington’s demand for a suitable replacement base for the troops that will remain on Okinawa — which it says is a prerequisite for the Marines’ relocation — has generated widespread opposition on Okinawa and talks have bogged down, throwing the whole plan into political limbo.