TAIPEI—Six Chinese fighter aircraft and a U.S. reconnaissance aircraft entered the southwestern corner of Taiwan’s air defense identification zone on Sunday, the island’s defense ministry said, in an unusual admission of U.S. military activity. Tensions have spiked over the last week or so after Taiwan reported multiple Chinese fighters and bombers flying into the zone last weekend, in an area close to the Taiwan-controlled Pratas Islands in the northern part of the South China Sea. The Chinese missions coincided with a U.S. aircraft carrier group entering the South China Sea for what the U.S. military termed a routine deployment. The United States has criticized the Chinese flights. Taiwan’s Defense Ministry said a total of seven Chinese aircraft flew into the same waters near the Pratas Islands on Sunday—two J-10 fighters, four J-11 fighters and a Y-8 reconnaissance aircraft. It added that a U.S. reconnaissance aircraft was also present in the …