US Secretary of State Hillary Clinton on Wednesday vowed military support for the Philippines, delivering a firm message from the deck of an American warship at a time of rising tensions with China.

On a steaming hot day on Manila Bay, Clinton boarded the USS Fitzgerald, a US Navy destroyer based in California, as she signed a declaration marking 60 years since the United States signed a security treaty with its former colony.

Clinton promised a wide-ranging commitment to the Philippines from military to economic cooperation, saying that the United States wanted to update its historic alliances to meet the "new challenges" of the 21st century.

"We must ensure that this alliance remains strong, capable of delivering results for the people of the Philippines and the United States and our neighbours throughout the Pacific," Clinton said.

Speaking later at a news conference, Clinton put the alliance in terms that many Filipinos might better appreciate -- praising boxing champion Manny Pacquiao, affectionately known in his native land as Pacman.

"I am a major Pacman fan," Clinton said. "In the spirit of the sport and his success, let me say, the United States will always be in the corner of the Philippines and we will stand and fight with you."

Clinton only indirectly mentioned China, which the Philippines and Vietnam accuse of increasingly aggressive tactics in the South China Sea, and said that the United States did not take any position on territorial disputes.

But Clinton referred to the South China Sea by her hosts' preferred name -- the West Philippine Sea -- and said that the United States wanted to assist Manila in defending its maritime boundaries.

"Any nation with a claim has a right to exert it, but they do not have a right to pursue it through intimidation or coercion," she said.

Philippine Foreign Secretary Alberto Del Rosario, speaking alongside Clinton on the USS Fitzgerald, said that Clinton's visit and the accompanying statement sent a strong signal on the disputes.

The statement "attests to the vitality of our alliance, especially at a time when the Philippines is facing challenges on its territorial integrity in the West Philippine Sea", he said.

The joint declaration between the two countries marking 60 years of relations also called for "freedom of navigation, unimpeded lawful commerce and transit of people across the seas".

Echoing a theme that the United States will likely push at a weekend East Asia Summit in Bali, the statement called for a "rules-based approach in resolving competing claims in maritime areas".

The US military presence is sensitive in the Philippines due to the colonial legacy, and a small number of left-wing activists protested Clinton's visit, accusing the United States of using the country for its own ends.

But amid the disputes with China, President Benigno Aquino has called for expanded military cooperation with the United States that is focused mainly on containing Islamic extremists in the remote southern Philippines.

Clinton later entered talks with Aquino, who is working to upgrade the Philippines' notoriously outdated military that features a navy made up mostly of retrofitted World War II-era ships from the United States.

Clinton also signed a pact bringing the Philippines into an initiative called the Partnership for Growth, which directs the US government to find ways to boost trade and investment with the next round of emerging economic powers.

Clinton will head later Wednesday to Thailand -- the other treaty-bound US ally in Southeast Asia -- in a show of support for Prime Minister Yingluck Shinawatra as she faces the daunting task of major floods.

Yingluck is the sister of former prime minister Thaksin Shinawatra, who was ousted in a 2006 coup that set off chaos in the kingdom. Washington has been concerned that instability in its oldest Asian ally would pose new challenges for its Asia policy.