Japan's centre-left Prime Minister Yukio Hatoyama said Monday that a local election victory by an opponent of a planned US military base on Okinawa island reflects the "will of the people".

The outcome of the mayoral vote Sunday makes it less likely that a controversial new Marine Corps air base will be built in a coastal area of the southern island on which tens of thousands of US troops are already stationed.

The base issue has soured ties between Washington and Tokyo since Hatoyama's government swept to power in August promising a less subservient stance towards the United States and announcing a review of the plan.

Both countries agreed in 2006 to move the air operations of the locally unpopular Futenma base from a crowded urban area to a quieter location in the coastal city of Nago by 2014.

Hatoyama, whose coalition includes a strongly pacifist party, has said he will decide by May whether to stick with the pact or move the base to another site, either on or off the island.

The United States has insisted that Japan honour the deal, which is part of a wider plan to realign its 47,000-strong troop presence in a country where American forces have been based since the end of World War II.

In Sunday's vote in Nago, the anti-base candidate Susumu Inamine, who was backed by Hatoyama's ruling party, beat the pro-base incumbent Yoshikazu Shimabukuro by 17,950 to 16,362 votes.

Hatoyama on Monday told reporters that "the election result shows the will of the people of Nago city", although he reiterated that the central government's final decision would not come before May.

Election winner Inamine, 64, said that once he takes up his post next month he will push for a city assembly resolution opposing the hosting of the new base, and later seek to hand over the resolution in person to Hatoyama.

Top government spokesman Hirofumi Hirano congratulated Inamine on his poll win but stressed that the local election would not prevent the possibility of Tokyo sticking with the original relocation plan.

Japan's media saw the local vote as a litmus test in the base row.

The liberal Asahi daily said that "the prime minister now needs to work his hardest to find a new candidate site outside" Okinawa.

The conservative Sankei daily criticised Hatoyama for "leaving the backbone of Japan-US national security up to the outcome of a mere mayoral election".

Under the 2006 pact, struck when the conservative Liberal Democratic Party were in power in Tokyo and the Republicans in Washington, the Futenma base would be closed and its air operations moved to a new facility at the Camp Schwab site at Henoko in Nago.

About 8,000 Marines would also be shifted to the US territory of Guam.

The base row and Hatoyama's at-times contradictory statements on it appear to have hurt his government's popularity.

Support for the Hatoyama cabinet has slipped by 6.6 points in two weeks to 46.4 percent, while the disapproval rating rose 6.9 points to 53.1 percent, according to a survey of 1,200 voters by private broadcaster TBS.

Toshio Kamo, politics professor at Ritsumeikan University, said "the election result certainly spells a tailwind for Mr. Hatoyama's pledge to remove US bases off Okinawa.

"On the other hand, the United States is intensifying pressure on Japan to accept the existing deal and some in Japan are getting concerned about strained ties with Washington," he told AFP.

"So Mr. Hatoyama is being sandwiched by pressure from residents and Washington. But if the premier truly takes initiative to resolve the issue and calls on President Obama again for a new solution, he could make a difference."