(Adds joint U.S.-Japan drill in Philippine Sea)
TAIPEI, Jan 23 (Reuters) - Taiwan on Sunday reported the largest incursion since October by China's air force in its air defence zone, with the island's defence ministry saying Taiwanese fighters scrambled to warn away 39 aircraft in the latest uptick in tensions.
Taiwan, which China claims as its own territory, has complained for more than a year of repeated missions by China's air force near the democratically governed island, often in the southwestern part of its air defence identification zone, or ADIZ, close to the Taiwan-controlled Pratas Islands.
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 latest Chinese mission included 34 fighters plus four electronic warfare aircraft and a single bomber, the Taiwan ministry said.
The aircraft flew in an area to the northeast of the Pratas, according to a map the ministry provided.
Taiwan sent combat aircraft to warn away the Chinese aircraft, while missile systems were deployed to monitor them, the ministry said.
There was no immediate comment from China, which has in the past said such moves were drills aimed at protecting the country's sovereignty.
While it was not clear what might have prompted China to carry out such a large-scale mission, U.S. and Japanese naval forces have been carrying out exercises in the Philippine Sea, a vast area that includes waters to the east of Taiwan.
The U.S. Pacific Fleet said on Sunday those exercises included two carrier strike groups who were "conducting training to preserve and protect a free and open Indo Pacific".
China has stepped up pressure on Taiwan to accept its sovereignty claims. Taiwan's government says it wants peace but will defend itself if attacked.
Taiwan reported 148 Chinese air force planes in the southern and southwestern part of its air defence zone over a four-day period beginning on Oct. 1, the same day China marked a key patriotic holiday, National Day.
Taiwan has reported almost daily Chinese air force forays into the same air space since then, but the number of planes on Sunday was the largest on a single day since the October incursions.
No shots have been fired and the Chinese aircraft have not been flying in Taiwan's air space, but in its ADIZ, a broader area Taiwan monitors and patrols that acts to give it more time to respond to any threats. (Reporting by Ben Blanchard Editing by Gareth Jones and Susan Fenton)