Tokyo, Japan (CNN) -- Japanese Prime Minister Yukio Hatoyama went back on a campaign promise and announced Sunday that a U.S. military base would remain in Okinawa.

He called his decision "heartbreaking."

"It is true that I said I wanted to relocate the facility outside of Okinawa," he said. "However, I'd like to apologize that the conclusion is not what the Okinawans wanted."

The U.S. Marine Corps Futenma base will be relocated to the Henoko area of the island, which is less densely populated, Hatoyama said.

Okinawa Gov. Hirokazu Nakaima demanded a better explanation from the government.

"I must say the situation is extremely disappointing and severe," he said.

The issue is an emotional one for Okinawans, who currently give up 10 to 20 percent of the island to the U.S. military.

While campaigning for Japan's top job last year, Hatoyama promised to move the base off of Okinawa altogether.

The island's residents, energized by an anti-U.S. campaign pledge in the last election, voted overwhelmingly for Hatoyama.

But as prime minister, Hatoyama found the promise difficult to keep, prompting residents to demand that he fulfill his pledge. Earlier this month Hatoyama visited the island and said it would be challenging to move a U.S. base off Okinawa.

The Futenma relocation is part of a 2006 agreement between Japan and the United States. Japan's delay in moving the base strained the 50-year alliance between the two nations.

The U.S. has said its military presence in Japan plays an important role in maintaining stability in the region. And the issue of relocating the base was high on U.S. Secretary of State Hillary Clinton's agenda during her visit to Japan last week.

Okinawans say the U.S. military has been responsible for a number of blights in Okinawa, from serious crimes like rape and drunken driving, to environmental and noise pollution.

Nearly 100,000 residents held rallies in April to demand that the base be moved off the island.

A recent Nikkei newspaper poll said that 59 percent of Japanese believe the prime minister should resign if he can't resolve the fight over the future of the Futenma military base.

CNN's Kyung Lah contributed to this report.