The United States will consider joint patrols with the Philippines in the South China Sea as part of efforts to ensure freedom of navigation in waters claimed by China, the US ambassador said Wednesday.

Washington angered Beijing in recent months by sending US warships and warplanes on patrols near artificial islands built by Beijing to bolster its claim to most of the Sea.

The Philippines, one of five claimants to parts of the waterway, has backed Washington's military actions and last month suggested joining US patrols.

"The United States... will continue to enjoy our rights under international law to sail through international waters or fly through international airspace," ambassador Philip Goldberg told reporters.

"So I'm not going to prejudge what we're going to do or when we're going to do it. Whether we're going to do it with the Philippines... I'm not discarding that possibility."

Asked later if he thought the United States would say "yes" if the Philippines formally asked to take part in joint patrols in the South China Sea, Goldberg said: "I think so."

He added: "I don't think there are any limitations on what the US can do."

Tensions flared between the two military superpowers late last year when the US flew two B-52 bombers close to flashpoint islands that have been artificially constructed by China.

In October a US guided missile destroyer sailed within miles of the area.

And last weekend, another US warship passed within 12 nautical miles of an island claimed by China, Taiwan and Vietnam, prompting Beijing's condemnation.

Beijing has asserted its ownership claims by rapidly building the artificial islands in the Spratly islands.

Port facilities, airstrips and military buildings have gone up on the man-made islands, prompting US warnings it would assert its rights to "fly, sail and operate wherever international law allows".