BEIJING — China's first aircraft carrier swept through fog-shrouded waters Wednesday to open sea trials that underscore concerns about the country's growing military strength and its increasingly assertive claims over disputed territory.

The mission by the refurbished former Soviet carrier marks a first step in readying the craft for full deployment. China says the ship is intended for research and training, pointing to longer-term plans to build up to three additional clones of the carrier in China's own shipyards.

"As a major economy, China on the one hand should take more responsibilities for the world and on the other hand, it has some new security interests that it needs to protect. Under the circumstances, China's naval power needs to grow accordingly," said Wang Shaopu, director of the Center for Pan-Pacific Studies at Jiaotong University in Shanghai.

Information about the cruise was tightly restricted in line with the Chinese military's habitual secrecy, although the official Xinhua News Agency indicated that the step had been planned for some time. The 1,000-foot (300-meter) vessel departed through fog from the northern port of Dalian where it is being overhauled.

"After returning from the sea trial, the aircraft carrier will continue refit and test work," Xinhua said.

The United States on Wednesday urged greater openness from China about its military capabilities and asked for a formal explanation of how the aircraft carrier would be used.

"China is not transparent as other countries, it's not as transparent as the U.S. about its military acquisitions, its military budget. This causes concern," State Department spokeswoman Victoria Nuland told a news conference in Washington.

"We are prepared to be extremely transparent with regard to U.S. military positions and equipment, and we would like to have a reciprocal relationship with China," she said.

China has spent the better part of a decade refurbishing the carrier formerly known as the Varyag after it was towed from Ukraine in 1998, minus its engines, weaponry, and navigation systems.

Beijing's carrier program is seen as the natural outgrowth of the country's burgeoning military expansion, fed by two decades of near-continuous, double-digit percentage increases in the defence budget. China's announced military spending rose to $91.5 billion last year, the second highest in the world after the United States.

While the development of carriers is driven largely by bragging rights and national prestige, China's naval ambitions have been brought into focus with its claims to disputed territory surrounding Taiwan and in the South China Sea.

Taiwan, the self-governing island democracy claimed by China as its own, has responded to the growing Chinese threat by developing missiles capable of striking carriers at sea. An illustration at a display Wednesday of military technology in the capital Taipei showed a Hsiung Feng III missile hitting a carrier that was a dead ringer for the former Varyag.

Over the past year, China has seen a flare-up in spats with Japan, the Philippines and Vietnam and had its relations strained with South Korea -- all of which have sought support from Washington, long the pre-eminent naval power in Asia.

China defends its carrier program by saying it is the only permanent member of the United Nations Security Council that has not developed such vessels and that it has a huge coastline and vast maritime assets to defend. Beijing has also said its carriers would be employed in international humanitarian efforts, although the ex-Varyag's ski jump-style flight deck severely limits the loads its planes can carry.

As the world's second-largest economy, China says it lags behind smaller nations such as Thailand and Brazil, as well as regional rival India, which have purchased carriers from abroad.

While Chinese carriers could challenge U.S. naval supremacy in Asia, China still has far to go in bringing such systems into play, experts said. The U.S. operates 11 aircraft carrier battle groups and its carriers are far bigger and more advanced.

Wednesday's exercise was essentially a test of the ship's propulsion system, with preparations to launch and recover aircraft still a long way off, said Andrei Chang, editor of Kanwa Asian Defence magazine.

"This was really just for show. They still have a long way to go," Chang said.

The Xinhua report did not say how long the sea trial would last. But a statement posted on the website of the Liaoning Maritime Safety Authority said vessels will be barred from entering a small section of the sea off Dalian until 6 p.m. (1000 GMT) on Sunday.

Positioning a carrier off its coast would boost the range of China's naval aircraft, increasing their ability to hit U.S. bases in Japan, South Korea and possibly Guam.

Beijing is believed to be developing a carrier version of the Russian Su-33, dubbed the J-15, a step that has angered defence officials in Moscow who accuse China or stealing their defence technology.

Both the European Union and the United States ban weapons sales to China, leaving Russia as its main overseas arms supplier.

With Moscow's defence industry declining in production and innovation, Chinese leaders have taken to marrying old Soviet platforms with cutting-edge Chinese technology. The same approach has been taken with the space program, where a capsule based on the former Soviet Soyuz design has been reengineered using new technology.

In contrast to China's slew of new frigates, submarines, and other warships, the carrier will actually add little to the country's naval capabilities, according to Western analysts.

"At best, it could makes some waves in the South China Sea and intimidate the poorly equipped navies of Vietnam, Indonesia and the Philippines," said Jonathan Holslag of the Brussels Institute of Contemporary China Studies.

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