By J.R. Wu

ITU ABA, South China Sea (Reuters) - Taiwan held rescue drills on Tuesday off the coast of its sole outpost in the Spratly Islands of the South China Sea, but the biggest claimant in the disputed waters kept uncharacteristically quiet.

China and self-governed Taiwan seldom see eye to eye, but in responding to Taipei's latest assertion of sovereignty over Itu Aba, Beijing has avoided the harsh language it often directs at other claimants to the busy waterway.

China, Taiwan, the Philippines, Vietnam, Malaysia and Brunei claim parts or all of the energy-rich South China Sea, through which trillions of dollars in trade passes annually.

Taiwan and China both suffered setbacks to their claims in July, when an international tribunal ruled that China's historic boundary, the so-called nine-dash line, was invalid, and said Itu Aba was a rock, rather than a self-sustaining island entitled to a 200-km economic zone.

Experts say Beijing is largely content for Taipei to push its claims on Itu Aba, the largest natural feature in the Spratlys, because China views Taiwan as a breakaway province, to be taken back by force one day, if necessary.

So, while Chinese ships have confronted Malaysian, Philippine and Vietnamese vessels in the area, Taiwan's regular journeys to and from the lonely outcrop have gone unimpeded.

"Our supply transports have never encountered Chinese interference," Lee Chung-wei, minister of Taiwan's Coast Guard Administration, told reporters visiting Itu Aba for the rescue drills.

In the exercises, coast guard vessels and navy helicopters practised how to retrieve injured crewmen from a burning ship and transport them to Itu Aba's small port and hospital.

PROTECTING SOVEREIGNTY

Vietnam, which also claims Itu Aba, objected to the exercises, saying they violated its sovereignty and threatened peace and stability in the region, the Vietnamese foreign ministry said in a statement late on Tuesday.

Asked about the drills on Itu Aba, and whether Taiwan had an obligation to respect Chinese sovereignty there, China's foreign ministry underscored its desire for a unified approach.

"The Nansha Islands, including Taiping Island, are inseparable parts of China," spokesman Geng Shuang told a daily briefing in Beijing, using China's terms for the Spratlys and Itu Aba. 

"Chinese people on both sides of the Taiwan Strait have an obligation to jointly protect this ancestral property."

China's Taiwan Affairs Office added that it would be paying "close attention" to what Taiwan did related to Itu Aba. It did not elaborate.

Drills such as Tuesday's show Taiwan's determination to become an important player, said Ian Storey, a South China Sea scholar at Singapore’s ISEAS Yusof Ishak Institute, adding that Taiwan officials and experts had told him of frustration at being diplomatically marginalized in the South China Sea.

Taiwan treads a fine line between annoying friendly Southeast Asian neighbors and the United States, its sole political ally and arms supplier, by being a proxy for China’s sovereign interests, despite the risk of angering Beijing, he added.

"It is a tricky position that means they are broadly supportive of Beijing," Storey said. "They want to be seen as pushing their own Chinese claims, rather than Beijing's, even though they are essentially the same."

Taiwan officials say President Tsai Ing-wen kicked off plans for the drills in July, a month after Beijing cut official communication channels because Tsai, who leads the independence-leaning Democratic Progressive Party (DPP), refuses to commit to the "one China" principle that Taiwan is part of the mainland.

To maintain ties, Tsai must continue arguing that Itu Aba is an island, not a rock, and is not to be used by military forces of other nations, said Wu Shicun, head of the National Institute for South China Sea Studies.

"Doing so would likely mean that they would threaten China’s sovereignty in the South China Sea," said Chinese government adviser Wu. "If she steps over these red lines, I believe China will take responsive action."

(Additional reporting by Christian Shepherd and Ben Blanchard in BEIJING, Greg Torode in HONG KONG, Mai Nguyen in HANOI; Writing by Lincoln Feast; Editing by Clarence Fernandez)