The United States and Japan are locked in a dispute over Washington's demand that Tokyo shoulder more costs for water supplies and utilities at U.S. military bases in Japan, media reports said on Monday.

Washington has urged Japan to cover rising costs of boosting U.S. military capabilities in the Asia-Pacific region to deal with North Korea and China, Kyodo news agency quoted sources close to the matter as saying.

Japan, a key U.S. ally in Asia, has resisted the demand, the sources were quoted as saying without elaborating.

"It could cause a crack in the Japan-U.S. relationship," Kyodo quoted a senior Japanese Foreign Ministry official as saying.

Japanese and U.S. officials were not immediately available for comment.

Japan, plagued by bulging public debts, want to reduce the so-called "sympathy budget" it sets aside each year under bilateral agreements to help support nearly 50,000 U.S. military personnel in Japan.

For the fiscal year ending next March, Japan allocated 217.3 billion yen ($1.86 billion) for the sympathy budget, 115 billion yen to cover salaries for Japanese employees at the bases and 25.3 billion yen for electricity, water and gas.

The reports come just as Japan's government is struggling to find a way to extend support for U.S.-led operations in Afghanistan.

The leader of the opposition Democratic Party, which won control of the upper house of parliament in July, has vowed to oppose a bill to extend the support mission, which he says lacks a U.N. mandate and violates Japan's pacifist constitution.

A suspension, which looks inevitable, could sour relations between the United States and its Asian ally.

The Nikkei business daily reported on Saturday that Japan was about to inform the U.S. and Britain that it would pull its ships out of the Indian Ocean on November 1. The mission has supplied fuel to 11 coalition nations since 2001, according to the government.