The Chinese navy's two active aircraft carriers have been deployed for separate drills, in the contested South China Sea and the Pacific Ocean, and military observers say they could soon be operating together.

The Shandong - China's first domestically built carrier, commissioned two years ago - has left its home port in Sanya, Hainan province for "combat-oriented exercises" in the South China Sea, the official People's Daily reported on Sunday.

• China says its carrier group conducts exercise in South China Sea

Drills will include fighter jet landings, damage control and maritime search and rescue, according to the report, which did not disclose the location of the exercises or when they would take place.

Meanwhile its sister ship, the Liaoning, led its strike group through Japan's Miyako Strait into the western Pacific for drills on Wednesday last week. Japan sent its light aircraft carrier, the Izumo, and escort ships to tail and monitor the Chinese strike group.

The Liaoning was accompanied by a large destroyer, a frigate and a comprehensive supply ship and conducted flight operations involving helicopters and fighter jets in the East China Sea and Pacific Ocean, the Japanese defense ministry said.

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The drills come as the USS Ronald Reagan carrier group, which patrols the Pacific, is back at its home port in Japan for the Christmas and New Year break, while the USS Carl Vinson is sailing from the Indian Ocean and has just entered the western Pacific.

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It is not the first time the two Chinese aircraft carriers have conducted exercises at the same time - in May, the Shandong was in the South China Sea for training while the Liaoning held drills in the western Pacific.

Military observers say the People's Liberation Army is trying to get the two strike groups working together. That could improve the Chinese navy's ability to strike inland targets and would make multi-platform coordinated attacks possible.

The two strike groups could also put pressure on the same target, such as Taiwan, from different directions, according to naval expert Li Jie, who is based in Beijing.

Reporting on the two Chinese missions, state-run nationalist tabloid Global Times said it was "only a matter of time" before the warships could cover a wider area, at sea and in the air.

China is building a third aircraft carrier that is believed to be bigger and more advanced than the active warships. According to commercial satellite images, considerable progress has been made on the vessel at a Shanghai shipyard and its launch could be three to six months away.

The PLA conducted at least three live-fire drills in the South China Sea last week, and an American spy plane was said to have been sent over the no-entry zones beforehand.

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