North Korea in April attempted to send a satellite into orbit using a long-range rocket. Similar attempts preceded nuclear tests in 2006 and 2009 and it's now suspected North Korea is on the verge of a third test.
Cho Byung-jae, a spokesman for the South Korean Foreign Ministry, warned there would be grave consequences should North Korea go ahead with the test.
"If North Korea goes ahead with a nuclear test, it will be a clear breach of the April 19 presidential statement by the U.N. Security Council and therefore the Security Council will have to take new actions," he was quoted by the Yonhap news agency as saying.
The Security Council in its April statement said North Korea was in violation of resolutions banning ballistic technology.
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, but the April launch ended in failure.