Pyongyang had agreed to a moratorium on long-range missile tests and nuclear activity in exchange for food assistance from the United States. The deal collapsed, however, when North Korea tried to send a satellite into orbit using a long-range rocket in April.
A South Korean official told the Yonhap news agency on condition of anonymity Pyongyang would likely test a nuclear device in response to international pressure.
"North Korea will make a decision on conducting a nuclear test based on its political judgment," the official said.
Analysts this week said information taken from satellite imagery suggested North Korea was preparing for a test. The South Korean official added Pyongyang was "technically ready" for a test.
Glyn Davies, U.S. special envoy for North Korean policy, said food assistance could continue if North Korea shows it is committed to its international obligations regarding nuclear activity.
Nuclear tests in 2006 and 2009 were coordinated with rocket launches by North Korea.