The United States defied China Tuesday by sending a warship close to artificial islands the rising Asian power is building in disputed waters, prompting Beijing to summon the US ambassador and denounce what it called a threat to its sovereignty.

The USS Lassen passed within 12 nautical miles -- the normal limit of territorial waters around natural land -- of at least one of the formations Beijing claims in the South China Sea.

China's defence ministry said a destroyer-class ship and another frigate were dispatched to "warn" the US vessel, which Beijing said it had "shadowed".

Washington's long-awaited move appeared to escalate tensions over the strategically vital waters, where Beijing has rapidly transformed reefs and outcrops into artificial islands with potential military use.

China claims sovereignty over almost the whole of the South China Sea, raising concerns it could one day dictate who may transit its busy sea lanes.

Several neighbouring countries including the Philippines, a US ally, have competing claims and the dispute has raised fears of clashes in an area through which a third of the world's oil passes.

The US action was part of its "routine operations in the Sea in accordance with international law", an American official told AFP. "We will fly, sail, and operate anywhere in the world that international law allows."

China's deputy foreign minister Zhang Yesui summoned US Ambassador Max Baucus on Tuesday to announce that the USS Lassen had engaged in a "serious provocation," the official news agency Xinhua said.

"The Chinese government will resolutely safeguard territorial sovereignty and legal sea interests, and China will do whatever necessary to oppose deliberate provocation from any country," Zhang added.

A foreign ministry spokesman said that the ship had "illegally entered" waters near the islands.

Fiery state-run tabloid the Global Times hinted in an editorial that Beijing could respond more strongly if the US made similar trips in the future.

"We should first track the US warships. If they, instead of passing by, stop for further actions, it is necessary for us to launch electronic interventions, and even send out warships, lock them by fire-control radar and fly over the US vessels," it said.

It added that: "At present, no country, the US included, is able to obstruct Beijing's island reclamation in the region."

But despite the Chinese rhetoric, analysts said more such US manoeuvres could be expected.

Beijing's so far limited response showed that it had had "its bluff called", said Rory Medcalf, director of the international security programme at the Lowy Institute in Sydney.

"The US and its allies and partners should now help the Chinese leadership in saving face, by emphasising that freedom of navigation operations are normal, not extraordinary," he said.

- Balance of power -

China's defence ministry said in a statement it had sent a "Lanzhou" missile destroyer and another ship to warn the USS Lassen.

It added that the US actions had "damaged trust" and said China would take "all necessary measures" to maintain national security.

There have been repeated confrontations in the area between Chinese vessels and boats from some of its neighbours who assert rights to the waters, particularly the Philippines and Vietnam.

Both are members of the Association of Southeast Asian Nations, which has long called on China to negotiate a code of conduct in the region, as are fellow claimants Brunei and Malaysia. Taiwan also makes claims over part of the sea.

Manila has infuriated the world's second-largest economy by taking the dispute to a United Nations tribunal, and Philippine President Benigno Aquino said the US action demonstrated that "the balance of power says that there is not just a single voice that must be adhered to".

Beijing's reclamations have been seen as an attempt to assert its claims by establishing physical facts, but Aquino said: "There is no de facto changing of the reality on the ground."

Beijing has repeatedly said the construction work is mainly for civilian purposes.

But satellite images of the islands published by the Washington-based Centre for Strategic and International Studies show that Beijing has reclaimed millions of square metres of land in the Spratlys, known as Nansha in Chinese.

The pictures also show a host of facilities with the potential for military applications being developed, including as many as three runways -- at least one of them 3,000 metres (10,000 feet) long.

- 'Long overdue' -

The US, which is engaged in a foreign policy "pivot" to Asia, and China, which has the world's largest military and is expanding the reach of its navy, are jockeying for position in the Pacific.

The sail-by was "long overdue", said Bonnie Glaser, a senior China expert at CSIS, adding that the exercises "should be done quietly, regularly, and often".

"There should be no media fanfare," she added. "The way this has been handled has left the Chinese believing that the US is challenging its sovereignty rather than simply exercising freedom of the seas."