China responded on Thursday to growing international defiance of its new air-defense zone in the East China Sea both by sending advanced fighter jets to the area and trying to play down any threat of military retaliation—underlining the confusion and escalated tension over the territorial dispute.

The announcement by China's air force that it had sent fighters and an early-warning aircraft to patrol the zone came just a few hours after Japan and South Korea, following the U.S.'s lead, said their military aircraft had flown...