BEIJING >> China’s first aircraft carrier headed back to sea Tuesday for a second round of sea trials, the Defense Ministry said.

The as-yet unnamed carrier left the northern port of Dalian on Tuesday to conduct "technical research experiments," the ministry said in a brief statement.

The carrier is the former Soviet Varyag, towed from Ukraine in 1998 minus its engines, weaponry and navigation systems. It was dispatched on its first five-day sea trials in August following years of refurbishment.

China has kept the sea trials relatively low-key amid regional concerns about its growing military strength and increasingly assertive claims to disputed territory.

This latest round comes days after the start of the Chinese navy’s annual training in the Western pacific that involves sailing between islands in Japan’s Okinawa chain.

A statement issued by the Defense Ministry last week said the exercises were not directed toward "any particular country or objective." However, they drew considerable attention in the media in Japan, where defense experts are wary of the Chinese navy’s increasing presence in Japanese waters.

China says the carrier is intended for research and training, leading to speculation that it plans to build future copies.

While no major problems have been reported in making the 55,000-ton ski jump-style carrier seaworthy, Beijing is believed to be years away from being able to launch and recover aircraft from it as part of a carrier battle group.