"I once again warn North Korea to immediately halt (preparations for) a third nuclear test that would do all harm but no good," Democratic United Party (DUP) Rep. Park Jie-won said during the first meeting of the party's emergency council.
Last week, Park was elected as the DUP floor leader in the incoming National Assembly. Until a new party leader is selected at next month's national convention, he will also serve as acting party chief in his capacity as head of the emergency council.
Park also called on South Korea and the United States to try to reduce tensions with the isolated North.
After North Korea's unsuccessful rocket launch on April 13, concerns have grown that the communist regime could stage additional provocations. Experts have talked of the possibility of a nuclear test, more missile tests and border clashes.
Officials in Seoul have said the North appears to have completed preparations for a nuclear test, digging a new underground tunnel at its Punggye-ri test site in the country's northeast, though it is unclear if or when it will conduct such a test.
North Korea previously carried out two nuclear tests following failed rocket launches, first in 2006 and then in 2009.
Last week, the five permanent U.N. Security Council members -- the United States, Britain, China, Russia and France -- issued a joint statement urging Pyongyang to "refrain from further actions which may cause grave security concerns in the region, including any nuclear tests."
On Sunday, North Korea rejected that appeal and vowed to continue to push strongly for its nuclear and rocket programs.