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 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.

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.

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.