The People's Liberation Army (PLA) said its Liaoning and Shandong carrier formations "recently... completed combat training on the high seas and returned to their home ports safely".

The mission was "routine training organised in accordance with annual plans", the military said on an official social media account without disclosing the dates or precise locations of the manoeuvres.

The carriers conducted joint combat and systems training as well as reconnaissance, early warning, anti-strike, sea assault, air defence and night flight training, the PLA said.

State broadcaster CCTV published a video of sleek grey warplanes soaring into the air from the runway on the deck of one of the carriers.

Other clips showed two planes flying in close formation, as well as a helicopter, with navy personnel watching through binoculars.

Recent Chinese manoeuvres in the Pacific have caused particular concern in Japan, which said this month it detected the two aircraft carriers operating simultaneously in the Pacific for the first time, indicating China's intention to improve operational capacity in remote areas.

Tokyo also said Chinese fighters took off from the Shandong and flew within 45 metres (150 feet) of a Japanese military patrol plane.

Chinese officials disputed the account at the time, blaming the incident on the Japanese aircraft.

The PLA acknowledged on Monday that "during the drills, foreign warships and aircraft approached (the carriers) several times to conduct reconnaissance, tracking and surveillance".

"The formations maintained a high alert level and a responsive combat status throughout the process, organised combat takeoffs of carrier-based aircraft many times, and handled affairs professionally and steadily," it said.

China has ploughed billions of dollars into modernising its military in recent years, a trend that has unnerved some governments in East Asia even though Beijing claims its aims are peaceful.

The Liaoning and Shandong are China's only two carriers currently in operation, but a third -- the Fujian -- is undergoing sea trials.

A Taiwanese security official also said this month that Beijing had deployed its two aircraft carrier groups around the island in May.

The Communist Party has refused to rule out using force to seize control of Taiwan, a democratic, self-ruled island that Beijing insists is part of its territory.