Kurt Campbell, the assistant secretary of state for East Asian and Pacific affairs, met two lawmakers from the opposition Liberal Democratic Party (LDP) in Washington ahead of a trip through Asia that starts Sunday.

"(Campbell) said if the relocation is not decided by May, the relationship could be affected in many fields," Katsuei Hirasawa told a news conference in Washington broadcast by Japans NHK.

Campbell told the lawmakers he wants to see progress in the dispute over the next few weeks, Hirasawa and Taichi Yamamoto, another LDP parliamentarian, said.

Japanese Prime Minister Yukio Hatoyama, who won a landmark election victory last year promising a less subservient stance toward the United States, has launched a review of a deal on relocation of the Futenma airbase in Okinawa.

Japans centre-left prime minister, who has pledged to make the final decision by May, said his government would decide by the end of March on a basic plan for the relocation.

Washington says the base is vital to defending Japan, which relies heavily on the US for regional security and is home to some 47,000 US troops, of which more than half are based on Okinawa.

If implemented, a 2006 agreement between the two countries, signed when the LDP was in power, would move the base from the crowded residential district to a quieter coastal part of the island. But some allies of Hatoyama want it to be moved off Okinawa or even outside Japan.