Residents of Okinawa want the base moved entirely off the island
Japan's Prime Minister Yukio Hatoyama has said it will not be feasible to entirely remove a controversial US base from the island of Okinawa.
The US Marines' Futenma base is deeply unpopular with many residents and removing it had been a key election pledge of the prime minister.
But on a visit to the island, Mr Hatoyama said "realistically speaking, it is impossible" to fully relocate it.
The island is home to over half the 47,000 American troops based in Japan.
Mr Hatoyama, speaking on his first visit to Okinawa since becoming prime minister, said maintaining the base in some form was needed for national security, under Japan's post-war military alliance with the US.
"I really feel sorry as I visit here today that I must ask for the Okinawan people's understanding that part of the base operations would have to stay," he said.
He called on the Japanese people to be "willing to share the burden, because the bases are necessary for national security".
Located off the south coast of the Japanese mainland
Home to several US bases and more than 20,000 troops
Island administered by the US from 1945 until 1972
Many residents blame US bases for crime and noise pollution
Mr Hatoyama, who had promised to resolve the issue by the end of this month, was greeted by banner-waving protesters demanding the closure of the base as he arrived for his one-day visit.
He said that regardless of where the troops were moved there would be "critical voices from local people".
But the governor of Okinawa prefecture, Hirokazu Nakaima, told reporters Mr Hatoyama's plans went against the wishes of local people, and that he still had time to change his mind.
"There is still some time until the end of May. I want him to continue to consider this in line with his election pledges," he said.
Japan and the US agreed a deal in 2006 to reduce the US troop presence in Okinawa, under which Japan would contribute $6.1bn (£4bn) towards the cost of moving some 8,000 troops to the Pacific island of Guam.
The airfield at Futenma would be closed and replaced by a new base in Nago in Okinawa's less populated north.
The US has been staunchly opposed to any renegotiation of the deal, and the row has damaged bilateral ties.
It has also undermined support for Japan's centre-left government.
Mr Hatoyama took office in September, promising to steer Japan into a more independent relationship with the US - reassessing Okinawa was a central part of this pledge.
But his approval rating have plummeted in recent month to around 20%, partly over what has been seen as his indecision over Okinawa.
Last month, nearly 100,000 people staged a protest on the southern island, demanding that the base be removed.
Islanders have been angered by incidents involving US troops based there, including the 1995 rape of a 12-year-old Japanese girl by three US servicemen and a helicopter crash on a university campus in 2004.
Other complaints have focused on noise levels and objections to the US military use of Japanese land.
Analysts say Mr Hatoyama's handling of the bases issue could be critical ahead of elections for Japan's upper house of parliament in July.
Okinawa is the focal point of the security treaty between the US and Japan which has balanced military power in the north-east Asian region since the end of World War II.
Under the pact, Japan - which is prevented from maintaining a war-ready army by its pacifist constitution - subsidises the US military presence while the US guarantees Japan's security.