TAIPEI: Two U.S. aircraft carrier groups have entered the disputed South China Sea for training, the Department of Defense said on Monday as Taiwan reported a new Chinese air force incursion at the top of the waterway including a fearsome new electronic warfare jet.

The South China Sea and self-governing Taiwan are two of China's most sensitive territorial issues and both are frequent areas of tension between the United States and China.

U.S. Navy ships routinely sail close to Chinese-occupied islands in the South China Sea to challenge Chinese sovereignty claims, as well as through the Taiwan Strait, to Beijing's anger.

The U.S. Department of Defense said the two U.S. Navy Carrier Strike Groups, led by their flagships USS Carl Vinson and USS Abraham Lincoln, had begun operations in the South China Sea on Sunday.

The carrier groups will carry out exercises including anti-submarine warfare operations, air warfare operations and maritime interdiction operations to strengthen combat readiness, it said in a statement.

The training will be conducted in accordance with international law in international waters, the Department of Defense added, without giving details.

"Operations like these allow us to improve our combat credible capability, reassure our allies and partners, and demonstrate our resolve as a Navy to ensure regional stability and counter malign influence," it quoted Rear Admiral J.T. Anderson, commander of the strike group led by the USS Abraham Lincoln, as saying.

Both carrier groups were reported on Sunday by the U.S. Navy to have been exercising with Japan's navy in the Philippine Sea, an area that includes waters to the east of Taiwan.

The news of the U.S. operations coincides with Taiwan reporting https://www.reuters.com/world/asia-pacific/taiwan-reports-new-large-scale-chinese-air-force-incursion-2022-01-23 the latest mass incursion by China's air force into its air defence identification zone - 39 aircraft - in an area close to the Taiwan-controlled Pratas Islands in the northern reaches of the South China Sea.

Taiwan on Monday reported a further 13 Chinese aircraft in the zone, with one, an anti-submarine Y-8, flying through the Bashi Channel which separates Taiwan from the Philippines and connects the Pacific to the South China Sea, according to a map provided by Taiwan's Defence Ministry.

The ministry added that two Chinese J-16Ds took part in the mission, though kept close to China's coast, a new electronic attack version of the J-16 fighter designed to target anti-aircraft defences of the sort Taiwan would rely on to fend off an attack.

China has yet to comment, but has previously said such missions are aimed at protecting its sovereignty and to prevent external interference in its sovereignty claims over democratically-governed Taiwan.

Security sources have previously told Reuters that China's flights into Taiwan's defence zone are also likely a response to foreign military activity, especially by U.S. forces, near the island, to warn that Beijing is watching and has the capability to handle any Taiwan contingencies.

Taiwan calls China's repeated nearby military activities "grey zone" warfare, designed to both wear out Taiwan's forces by making them repeatedly scramble, and also to test Taiwan's responses.

The South China Sea, crossed by vital shipping lanes and also containing gas fields and rich fishing grounds, is also claimed by Taiwan, while Vietnam, Malaysia, Brunei and the Philippines claim parts.