Taiwan on Monday reported the largest incursion since January by China’s air force in its air defense zone, with the island’s defense ministry saying Taiwanese fighters scrambled to warn away 30 aircraft in the latest uptick in tensions.

Taiwan, which China claims as its own territory, has complained for the past two years or so of repeated missions by China’s air force near the democratically governed island, often in the southwestern part of its air defense 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.