North Korea this year agreed to a moratorium on long-range missile and nuclear tests in exchange for substantial food assistance from the United States. That deal collapsed last month, however, when North Korea tried to send a satellite into orbit using a long-range rocket.
Similar rocket and missile tests by North Korea in 2006 and 2009 coincided with the testing of a nuclear device. The failed April satellite launch was widely believed to be a prelude to a third nuclear test.
Park Ji-won, leader of South Korea's Democratic United Party, warned North Korea against another test.
"I once again warn North Korea to immediately halt [preparations for] a third nuclear test that would do all harm but no good," he was quoted by South Korea's Yonhap news agency as saying.
Acting as head of South Korea's national emergency council, he added that Seoul and Washington should work to reduce tensions in the Korean Peninsula.
His warning came as U.S. and South Korean forces launched joint military drills involved 60 war planes in a 12-day maneuver dubbed Max Thunder, China's official Xinhua news agency reports.
Pyongyang had threatened to retaliate if regional exercises were viewed as a threat.