Japan on Friday ended a naval refuelling mission that has supported the US-led military effort in Afghanistan since 2001 as the centre-left government flexes its muscle in its ties with Washington.
The move fulfils a pledge by Prime Minister Yukio Hatoyama's government which ousted the long-ruling conservatives four months ago pledging a less subservient relationship with the United States.
It comes days before Washington and Tokyo mark the 50th anniversary of signing their security alliance, which has been strained by a row over the relocation of a US military base on the southern island of Okinawa.
Defence Minister Toshimi Kitazawa ordered the two naval ships and their 340 personnel to return home after eight years of helping supply oil and water to vessels used by international forces that are engaged in Afghanistan.
"The defence minister issued an order ... today to the fleet commander to end refuelling activity in the Indian Ocean at 12:00 p.m. (1500 GMT) on January 15 and to send the troops home," a ministry statement said.
With the end of the refuelling mission, Hatoyama has pledged that Japan would instead step up humanitarian aid to Afghanistan. Tokyo has offered five billion dollars over the next five years to help rebuild the war-torn nation.
Hatoyama, whose coalition includes the strongly pacifist Social Democrats, has stressed Japan would not deploy troops to Afghanistan.
Under its post-war pacifist constitution, Japan is barred from sending armed forces overseas for combat, although Tokyo has deployed soldiers abroad for peacekeeping and military support missions in Iraq and elsewhere.
Japan's ties with the United States, its closest security ally, have been strained by the row over the base in Okinawa, an island where more than half of the 47,000 US troops in Japan are stationed.
Both countries agreed in 2006 that the locally unpopular Futenma Marine Corps Air Station would be moved from a crowded urban area to a quieter coastal region by 2010, but Hatoyama's government is now reviewing the deal.
Many local residents on the island, which was the scene of some of the bloodiest battles of World War II, have opposed the heavy US troops presence and complained of aircraft noise, pollution and crimes committed by US troops.
Hatoyama has said his government would decide by May where to relocate the base as it is considering alternative sites, with various options including moving it off Okinawa altogether, to Washington's chagrin.
This week US Secretary of State Hillary Clinton pressed her Japanese counterpart Katsuya Okada to stick by the original deal on moving the base, but got no commitment from Tokyo during the Hawaii meeting.
She reiterated the US view that the 2006 base deal known as the realignment road map is "the way forward" for the long-time allies.
The US-Japan security treaty, signed on January 19, 1960, has been the bedrock of the post-war alliance, under which pacifist Japan relies on a massive US military presence to guarantee its security.