China's only aircraft carrier has begun a second set of sea trials focusing on "relevant scientific research and experiments," China's Ministry of National Defense said.

The trials come as China's relations with its neighbors are increasingly strained over territorial disputes.

The ship, called the Varyag, had its first sea trial in August. Upon return to its home port of Dalian, the Ministry of Defense said "all systems were operating as normal."

The vessel, a former Soviet aircraft carrier, has been refitted for its new role as a research and training platform for the Chinese military, a report in the government-controlled news agency Xinhua said.

Ministry officials gave no other details of its second sea trials.

China bought the hull of the unfinished vessel in 1998, with no guns and engines, from the Ukrainian shipyard where it had been under construction. It was an unfinished project left over from the days when the Ukraine was part of the Soviet Union until its collapse in 1991.

The vessel remains officially unnamed in China but is referred to by its old name when mentioned in reports.

The vessel, an Admiral Kuznetsov class aircraft carrier, measures around 1,000 feet long and 122 feet wide, with a displacement of 58,500 metric tons.

Chinese officials have stressed since August that the vessel isn't aimed at offensive military operations and that other nations have similar ships.

"Currently, the United States, the United Kingdom, France, Russia, Spain, Italy, India, Brazil and Thailand, operate a total of 21 active-service aircraft carriers," Xinhua said.

In July China also confirmed that it has another aircraft carrier under production but gave few details.

A report in December 2008 by the Japanese newspaper Asahi Shimbun said China would begin construction of the 50,000-ton Shanghai, its first domestically built carrier, in 2009, for launch in 2015. It and the Varyag are to patrol the South China Sea.

Despite Beijing's reassurances that Varyag is only for scientific and training purposes, the vessel's sea trials come as China appears to be flexing its naval military muscles in the South China Sea, to the concern of neighboring countries with whom Beijing has territorial disputes.

China's maritime activities are "overbearing" and arouse "anxiety about its future direction," a Japanese Defense Ministry white paper states.

The document published in July and called "Defense of Japan, 2011," describes China's rapid modernization of its military hardware as an attempt to "strengthen its capacity to have its military potential reflected in distant locations."

Beijing's buildup of its maritime power and its defense policy in general, which was noted in last year's white paper, continues to be "a concern for the regional and the international community." China should "be aware of its responsibility as a major power and abide by international rules."

The 600-page document noted Japan is concerned about territorial disputes between China and its neighbors, in particular the dispute over the Spratly Islands, which "will affect peace and security in regional and global society."

The Spratly Islands -- the largest group -- lie off the southwest coast of the Philippines as well as Brunei and Malaysia. Ownership of the Spratly Islands is the most difficult of all the territorial claims because of the number of claimants, including Vietnam and Taiwan.

This week Chinese President Hu Jintao told deputies in the naval section of the Party Congress of People's Liberation Army that the military should "accelerate the transformation and modernization of the navy," Xinhua reported. The navy should "make extended preparations for warfare in order to make greater contributions to safeguarding national security and world peace," the report said.