TOKYO (Reuters) - Japan will put off a decision on relocating a U.S. airbase, the foreign minister said on Tuesday, which the top U.S. Marine officer called "unfortunate" and that risks fanning doubts about Prime Minister Yukio Hatoyama's leadership.

Tokyo is under U.S. pressure to implement a 2006 plan to relocate the Marines' Futenma airbase on Okinawa to a less crowded part of the southern island as a prerequisite for shifting up to 8,000 Marines to the U.S. island of Guam.

Hatoyama had originally intended to decide by year-end what Japan would do, but Chief Cabinet Secretary Hirofumi Hirano said the three parties making up the ruling coalition would continue to discuss the issue.

"On the issue of bases, we confirmed that the three parties would work together on the matter," Hirano told a news conference.

Asked what that meant, Foreign Minister Katsuya Okada told reporters later, "It means 'please wait a little longer.'"

He added the coalition would try to keep any effect on the alliance with Washington to a minimum.

The plan to move Futenma is part of a broader realignment of U.S. troops in Japan against a background of China's rising role and an unpredictable North Korea.

Marine Corps Commandant General James Conway, told about the development by reporters, said it could complicate the plan to move Marines to Guam by 2014.

"If that is their decision, then I think it's unfortunate in terms of what we're attempting to plan on our end," Conway said.

PRIME MINISTER SAYS DELAY NOT IRRESPONSIBLE

Hatoyama has vowed to steer a diplomatic course less dependent on Washington, Tokyo's long-standing security ally, and said during the August election campaign that swept him to office that he wanted to move the base off Okinawa.

Japanese media had reported that Washington wanted a decision on relocation by the end of the year.

Hatoyama denied the delay was irresponsible.

"Especially given the weight of the U.S.-Japan agreement, and on the other hand the strong feelings of the Okinawan people, even if we made an immediate decision, it would fall apart," he told reporters on Tuesday.

"I don't believe I have taken an irresponsible attitude at all," he added, vowing to find a solution acceptable to all.

Hirano declined to confirm reports the government had proposed a May 2010 deadline for a decision on relocating Futenma.

Kyodo news agency reported that the Social Democratic Party, a small coalition partner that wants the airbase moved off Okinawa entirely, had opposed setting a deadline.

An opinion poll by public broadcaster NHK on Monday showed approval for Hatoyama's government at 56 percent, down 9 points from last month, while disapproval was up 13 points at 34 percent.

The main factor behind the disapproval was Hatoyama's inability to get things done, the NHK poll showed.

An analyst said the impact on Hatoyama from the base dispute depended in part on the U.S. reaction, including whether Washington agreed to new talks.

"From the U.S. perspective, this (the 2006 deal) should have been maintained despite the change of government, so it would only be natural for them to feel betrayed," said Katsuhiko Nakamura, director of research at think tank Asian Forum Japan.

(Reporting by Chisa Fujioka, Linda Sieg, Isabel Reynolds in Tokyo and Phil Stewart in Washington; Editing by Peter Cooney)