TOKYO (AFP) - The USs top military officer on Friday stepped up pressure on Japan to settle a military base dispute while stressing the need to maintain their decades-long alliance, news reports and officials said. Admiral Michael Mullen, chairman of the US joint chiefs of staff, visited Japan as the security alliance is being strained by a row centred on the planned relocation of the Marine Corps Futenma Air Base on Okinawa island. Japans five-week-old centre left government has said it will review a 2006 agreement to move the base from a crowded urban to a coastal area of Okinawa by 2014, and has even suggested the facility be moved off the island. I see the agreement weve reached and the specific Futenma relocation issue as an absolute requirement, Mullen was reported by Kyodo News as saying. Foreign Minister Katsuya Okada, meanwhile, gave some ground in the debate, saying the base should not be moved off the island, but could be merged with other US military facilities a plan also not favoured by Washington. We cant think that moving (the base) out of the prefecture is a realistic option, Okada said. He added that he personally favoured the proposal to merge Futenma air operations with the existing Kadena Air Base, Americas biggest air base in Asia. The idea of moving Futenma operations there is expected to draw opposition from left-leaning government coalition partners and many Okinawans, who object to the US troop presence and complain of aircraft noise. Mullens visit came two days after US Defence Secretary Robert Gates bluntly pressed Japan to move on quickly with the previously agreed plans to move Futenma operations to the the coastal area near the US Camp Schwab site. Prime Minister Yukio Hatoyama on Friday suggested the issue may not be resolved before President Barack Obama visits Japan on November 12-13. When Mullen met Okada, the US admiral confirmed the significance of the Japan-US alliance and the need to overcome the current issues based on relations in trust, the Japanese foreign minister told reporters. The United States, which defeated Japan in World War II and then occupied the country, now has 47,000 troops stationed there, more than half of them on Okinawa, the site of one of the bloodiest battles of the war.