America's top military commander in the Pacific says the United States will maintain a presence in the South China Sea for many years, to ensure free navigation through its vital sea lanes.

Admiral Robert Willard, head of the U.S. Pacific Command, said China's increasingly "assertive" behavior in the sea was discussed during talks with Philippines officials in Manila on Wednesday.

A senior U.S. Defense Department official Deputy Assistant Secretary of Defense for South and Southeast Asia Robert Scher held similar talks with Vietnamese officials on Tuesday.

Willard said the United States is not taking sides in a dispute between China and several of its neighbors over potentially oil- and gas-rich islands in the South China Sea. But he said America will remain in the region to ensure free navigation through the waters, which carry more than half of Asia's seaborne trade.

China was angered when U.S. Secretary of State Hillary Clinton made similar remarks at an Asian security forum in Hanoi last month, declaring that the United States has a "national interest" in maintaining free navigation in the sea.

Within days, China announced it had conducted the largest naval exercises in the history of the People's Liberation Army in the waterway, under the personal direction of the PLA's chief of staff Chen Bingde.

In an annual report mandated by the U.S. Congress, the United States said Monday that China is developing new military capabilities enabling it to influence regional disputes, expand its territorial waters and deny adversaries the use of sea lanes.

A Chinese Defense Ministry spokesman Geng Yansheng disputed that in a statement faxed to the French news agency Wednesday. He said the report was "not beneficial" to improved military ties between the United States and China, and maintained that his country's defense policy "does not pose a threat to any country."

U.S. concern has been growing since March, when Chinese authorities told two visiting American officials Deputy Secretary of State James Steinberg and National Security Council Asia director Jeffrey Bader that Beijing considers the South China Sea a "core national interest." The phrase has previously been applied to Taiwan, Tibet and Xinjiang, regions that China has expressed a willingness to defend with military force.

At the end of last month, a Chinese Defense Ministry spokesman Senior Col. Geng Yansheng was quoted in the Washington Post saying China has "indisputable sovereignty" over the South China Sea but that it would allow other countries to navigate through its waters.

The United States, for its part, last week held its first naval training exercises with Vietnam, which also has claims in the sea. Others with claims to the disputed Spratly Islands are the Philippines, Malaysia, Brunei and Taiwan.

The United States has also been conducting naval exercises off the coast of South Korea which it says are intended to discourage North Korean aggression. China has warned Washington against naval operations in the Yellow Sea.

Some information for this report was provided by AFP, AP and Reuters.