UPDATE 1-US backs Philippines as S.China Sea tensions flare

* Clinton reasserts U.S. interests in S. China Sea

* U.S. pledges to honor defense treaty with Philippines

* Philippines worried over 'increasingly aggressive' China (Recasts with Clinton, del Rosario quotes; adds byline)

By Andrew Quinn

WASHINGTON, June 23 (Reuters) - U.S. Secretary of State Hillary Clinton on Thursday pledged to support the Philippines amid growing tensions between China and its neighbors in disputed areas of the South China Sea.

Clinton -- brushing aside a Chinese warning for the United States to stay out of the dispute -- said U.S. national interests in freedom of navigation and respect for international law were at stake.

"We are concerned that recent incidents in the South China Sea could undermine peace and stability in the region. We urge all sides to exercise self restraint, and we will continue to consult closely with all countries involved including our treaty ally the Philippines," Clinton said in a joint appearance with Philippine Foreign Affairs Secretary Albert del Rosario.

"The United States does not take sides on territorial disputes over land features in the South China Sea, but we oppose the use of force or the threat of force to advance the claims of any party," she said.

China has become increasingly assertive in pressing its claim to the entire South China Sea, believed to be rich in oil and gas, and Vietnam has accused Chinese boats of harassing a Vietnamese oil exploration ship in the region.

Other parts of the South China Sea also are claimed by Brunei, Malaysia, the Philippines, Taiwan and Vietnam.

Clinton last year staked out the U.S. position on the South China Sea, declaring that the United States had its own interests in ensuring conflicts are handled peacefully, in what was seen as a diplomatic challenge to Beijing.

BEEFING UP THE NAVY

Del Rosario had earlier said that the Philippines was counting on unwavering U.S. help to beef up its naval capabilities, and Clinton said that Washington would stand by its old friend.

"I want to underscore our commitment to the defense of the Philippines," Clinton said, saying the United States would honor both its mutual defense treaty and its long-standing strategic alliance with the Southeast Asian nation.

Manila had sought a clarification from Washington of its position on how it would respond to a situation in the South China Sea under the two countries' 1951 Mutual Defense Treaty, as well as help upgrading its naval capabilities.

The Philippines, which had long received reconditioned surplus U.S. military equipment, was examining new arrangements such as leases that would offer newer gear delivered more quickly, del Rosario said in an earlier appearance in Washington.

Clinton said, "We are determined and committed to supporting the defense of the Philippines and that means trying to find ways of providing affordable material and equipment."

The dispute between Manila and Beijing flared again after Chinese navy ships and a marine surveillance vessel were seen placing a buoy and posts near a bank in the sea in an area Manila said was well inside the Philippines' Exclusive Economic Zone. Del Rosario said China's intrusions were "clearly becoming more aggressive and more frequent".

"We of course are very concerned about where the direction of these events may be headed," he said.

Chinese Vice Foreign Minister Cui Tiankai on Wednesday told foreign reporters in Beijing that China had not provoked any incidents in the South China Sea and said if Washington wanted to play a role it should urge restraint on other claimants.

"I believe the individual countries are actually playing with fire and I hope the fire will not be drawn to the United States," said Cui, who will hold talks in Hawaii later this week with U.S. Assistant Secretary of State Kurt Campbell.

(Additional Reporting by Paul Eckert; Editing by Eric Walsh)