Japan's Social Democratic Party (SDP) says it will quit the ruling coalition if the government decides to keep a US military base in Okinawa.
SDP leader Mizuho Fukushima said on Thursday that "the Social Democratic Party and I will have to make an important decision" if the government decides to go ahead with the plan.
Tokyo is under pressure from Washington to implement a 2006 Japan-US deal, under which the military base would be replaced by a new one to be built elsewhere on Okinawa Island.
As a junior partner in the three-way coalition government, the SDP has advocated relocating the base elsewhere in Japan or overseas.
Since the new government took office, Prime Minister Yukio Hatoyama has formed a coalition with the pacifist Social Democrats, whose support he needs to pass legislation.
The premier noted that he would take the SDP's views seriously, and added, "Finding a solution in this situation will be no easy matter, but we must work hard."
Washington has about 47,000 troops based in Japan, more than half of them in Okinawa. Local residents have been angered by crimes committed by the US service personnel as well as the risk of accidents.
In 1995, the rape of a schoolgirl by three US servicemen infuriated residents of Okinawa. Demands to close the base on safety grounds rose when a US helicopter crashed in the grounds of a local university in 2004.
Japanese media reports predicted on Thursday that Hatoyama was likely to postpone the decision until next year.