TOKYO: Japanese voters in a city on Okinawa island started casting ballots Sunday in a mayoral vote that throws a spotlight on a row with the United States over where to build a controversial new US air base.

The two candidates in Nago city are squaring off over whether or not to give local support to a plan -- currently under review by the centre-left national government -- to build a major new Marine Corps air base there.

The issue has strained ties between Tokyo and Washington, who marked the 50th anniversary of their security pact last Tuesday, since Japan's new leaders took power four months ago ending a half-century of conservative rule.

Prime Minister Yukio Hatoyama has said he may scrap an agreement with Washington to relocate the base from its current site in a crowded urban area of Okinawa to a quieter coastal site in the Nago area by 2014.

In Sunday's mayoral race, which polls have said is too close to call, the incumbent Yoshikazu Shimabukuro, 63, supports building the replacement base in Nago while his rival Susumu Inamine, 64, opposes it.

The southern island of Okinawa, which saw some of the bloodiest battles of World War II, now hosts more than half of the 47,000 US troops in Japan.

While some local businesses benefit from the heavy American military presence, many residents have long opposed it, citing crimes committed by servicemen as well as noise, pollution and the threat of accidents.

Hatoyama, whose coalition government includes pacifist groups and stern opponents of the US military presence, has said he will make a decision by May on where to move the controversial Futenma air base.

Some analysts speculated that Hatoyama would ultimately settle with the current plan as there is no other realistic option for the replacement base in Japan, with no other communities willing to host it.

If the candidate against the base is elected, the central government can technically overrule a local government decision to refuse the base. But such a move could come with sharp criticism as Hatoyama has vowed to consider "pains inflicted on Okinawans" and to respect local governments' decisions.

Some observers have voiced frustration with the premier for making at times contradictory statements on the issue, leaving both Okinawans and Washington officials confused about his intentions.

"What Mr. Hatoyama has said so far is inconsistent, and I don't know how he wants to settle the base row," said Yoshinobu Yamamoto, professor of international politics at Aoyama Gakuin University.

Yamamoto pointed out that, as an opposition politician years ago, Hatoyama advocated the withdrawal of all permanent US forces from Japan.

Hatoyama last month said that his views had changed but added that "there is an argument about whether it is appropriate to have foreign troops stationed in Japan if you think about the future, 50 or 100 years from now."

Yamamoto said: "If he truly believes in an 'alliance without the permanent presence of US troops' in Japan, there may be fundamental differences in the perception of the alliance between him and the United States.

"It could be a disaster for the future of the alliance."

A group of liberal academics last week supported Hatoyama's review of the base agreement, called for a review of the Japan-US alliance and said: "We should also make efforts to remove other (US) bases in the future."