Hidenao Nakagawa, the secretary general of Japan's ruling Liberal Democratic Party, said North Korea was not yet a nuclear power if its test last week was a failure, as suggested by US experts.
"If it was a failure, then North Korea does not possess a nuclear capability," Nakagawa said, urging Pyongyang to abandon its nuclear ambitions.
"If there are second or third tests, the North Korean leadership will have to take responsibility for things that happen as a result," he told a news conference, suggesting further sanctions on the North.
Nakagawa, considered a hawk on foreign policy, is known for his close relationship with Prime Minister Shinzo Abe, who took office last month.
Japanese Foreign Minister Taro Aso earlier said he had "information" North Korea was planning a second nuclear test, while Washington warned Pyongyang against another "provocative" act.
The United States said Monday that the tests of air samples confirmed that North Korea had carried out its first-ever atomic explosion last week, but said the test was less than one kiloton -- indicating it may have been a failure.
North Korea's announcement of a test led to world-wide outrage, including a UN Security Council resolution imposing sanctions on the communist state.
Japan has also imposed its own sweeping sanctions, banning all imports and ships from North Korea.

Copyright AFP (Agence France-Presse), 2006