By George Nishiyama

TOKYO, Oct 9 (Reuters) - Japan should provide support for NATO-led troops in Afghanistan, but pull out of a naval mission in the region, a senior lawmaker of the largest opposition party said on Tuesday.

The Democratic Party -- the biggest force in the upper house -- has vowed to vote down the ruling bloc's plan to continue Japan's contribution to the U.S.-led operation, a move that could sour relations with Washington.

Prime Minister Yasuo Fukuda is seeking to extend the naval mission in the Indian Ocean beyond Nov. 1, when it is due to expire, arguing that stopping it would prompt criticism that Japan had abandoned its role in combating terrorism.

But Kenji Yamaoka, chief of parliamentary affairs for the Democrats, said the party was against the mission, which provides fuel and water to ships policing the Indian Ocean, because it lacked a U.N. mandate.

"There is no need to take part in America's war," Yamaoka, also a close aide to party leader Ichiro Ozawa, told Reuters.

"But we're more than willing to provide materials and human resources for ISAF, which is a peace-keeping mission," he added, referring to the U.N.-authorised International Security Assistance Force in Afghanistan.

A vote to suspend the Indian Ocean mission now appears inevitable, although the ruling bloc can override the upper house with its two-thirds majority in the more powerful lower house, enabling Japanese ships to resume operations.

But Yamaoka said plenty of other measures would better contribute to stabilising Afghanistan, where ISAF's nearly 40,000 soldiers are fighting the country's former Taliban rulers, ousted by U.S. and Afghan forces in 2001.

He said logistical support for ISAF, such as supplying fuel, was possible, but added the party had not finished its proposal.

The government has said Japan cannot take part in ISAF, given its pacifist constitution. Analysts have also said Japanese participation would not be realistic, as ISAF not a peacekeeping mission but is rather fighting a war against Taliban insurgents.

Security conditions in Afghanistan have deteriorated in the past 20 months and over 700 ISAF members have been killed.

But Democrat leader Ozawa has said taking part in ISAF would be in line with the constitution's preamble, which calls on Japan to contribute to international efforts to secure global peace.

Japanese voters are split on whether to continue the Indian Ocean mission. A weekend survey by broadcaster NHK showed 25 percent of respondents supporting it and 21 percent opposed.

The LDP's July election defeat gave the opposition a majority in the upper house -- and the ability to delay most legislation.

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