By Greg Torode

HONG KONG (Reuters) - U.S. plans to send warships or military aircraft within 12 nautical miles of China's artificial islands in the disputed South China Sea, possibly within days, could open a tense new front in Sino-U.S. rivalry.

A range of security experts said Washington's so-called freedom of navigation patrols would have to be regular to be effective, given Chinese ambitions to project power deep into maritime Southeast Asia and beyond.

But China would likely resist attempts to make such U.S. actions routine, some said, raising the political and military stakes. China's navy could for example try to block or attempt to surround U.S. vessels, they said, risking an escalation.

Given months of debate already in Washington over the first such patrol close to the Chinese outposts since 2012, several regional security experts and former naval officers said the U.S. government might be reluctant to do them often.

U.S. allies such as Japan and Australia are unlikely to follow with their own direct challenges to China, despite their concerns over freedom of navigation along vital trade routes, they added.

"This cannot be a one-off," said Ian Storey, a South China Sea expert at Singapore's Institute of Southeast Asian Studies.

"The U.S. navy will have to conduct these kinds of patrols on a regular basis to reinforce their message."

The Obama administration has said it would test China's territorial claims to the area after months of pressure from Congress and the U.S. military. It has not given a timeframe.

"I think we have been very clear - that we intend to do this," State Department spokesman Mark Toner told reporters last Monday.

Chinese Foreign Ministry officials said this month that Beijing would "never allow any country to violate China's territorial waters and airspace in the Spratly islands in the name of protecting navigation and overflight".

Under the U.N. Convention on the Law of the Sea, 12-nautical mile limits cannot be set around man-made islands built on previously submerged reefs.

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Four of the seven reefs China has reclaimed over the last two years were completely submerged at high tide before construction began, legal scholars say.

China claims most of the South China Sea. Other claimants are Vietnam, the Philippines, Malaysia, Brunei and Taiwan.

"NO-GO ZONE"

Bonnie Glaser, a security expert at Washington's Center for Strategic and International Studies, said U.S. missions would likely be regular, with the navy wanting to ensure it did not become effectively shut out of the area.

"I know the U.S. does not want that outcome. Nobody wants to give the Chinese a new no-go zone and an effective territorial sea they are not entitled to," she said.

Glaser said she believed China would be careful about interfering with a U.S. patrol, despite past frictions.

Myles Caggins, a spokesman for the White House National Security Council, declined to comment when asked whether a U.S. show of force might be more symbolism than substance unless there was a sustained naval effort, or whether the administration was factoring in further Chinese assertiveness.

He said U.S. thinking was illustrated by President Barack Obama's statement at a news conference with Chinese President Xi Jinping in Washington last month that "the United States will continue to sail, fly and operate anywhere that international law allows".

Despite Xi's comment at the news conference that the man-made islands would not be militarised, some mainland Chinese analysts believe the reclamations will form the heart of a new military screen protecting Chinese submarines on southern Hainan Island, as well as boasting extensive civilian facilities.

These submarines will soon carry nuclear weapons and represent the core of China's nuclear deterrence, giving it a second strike capability.

"DANGEROUS ESCALATION"

While China's outposts are seen as vulnerable in a conflict, up until that point they will allow Beijing to extend both civilian activities, such as fishing and oil exploration, as well as military patrols. One airstrip is finished and two others are being built.

Zhang Baohui, a Chinese security expert at Hong Kong's Lingnan University, said he feared a "dangerous escalation", with China likely to react to any attempt to make the patrols routine.

Rather than freedom of navigation, Zhang said he believed Beijing saw the issue as one of great power rivalry.

"It is all about power, and that is what makes this so dangerous," he said.

China had never formally declared a 12-mile territorial zone around the reclamations, so any U.S. show of force was premature, he added.

Sam Bateman, an adviser to Singapore's S. Rajaratnam School of International Studies and a former Australian naval officer, also noted the lack of any formal declaration, adding Washington risked underestimating China's angst over being contained in the South China Sea.

"There is a real risk of a confrontation between China and the U.S. that the U.S. might have to withdraw from," he said, urging more diplomacy instead.

"I'm not sure what their end-game is."

(Reporting by Greg Torode in HONG KONG; Additional reporting by Matt Spetalnick and David Brunnstrom in WASHINGTON; Editing by Dean Yates)