South Korea's U.N. ambassador said Monday a North Korean nuclear test "seems to be imminent."

Ambassador Kim Sook said there are "very busy activities" taking place at North Korea's nuclear test site "and everybody's watching."

Kim told a press conference that in the event of a nuclear test, he expects the U.N. Security Council to respond with "firm and strong measures."

North Korea announced last month that it would conduct a nuclear test to protest Security Council sanctions toughened after a satellite launch in December that the U.S. and others say was a disguised test of banned missile technology. The council ordered North Korea in the sanctions resolution to refrain from a nuclear test or face "significant action."

South Korea joined the Security Council in January and holds the rotating presidency this month. Kim said he was speaking as South Korea's ambassador, not as the council president.

He said that during negotiations on the latest sanctions resolution all 15 council members _ including North Korean ally China _ were unified.

"They are very firm and resolute and I would expect very firm and strong measures to be taken in terms of format as well as in substance once they go ahead with such provocation" as a nuclear test, Kim said.

Pyongyang's two previous nuclear tests, in 2006 and 2009, both occurred after it was condemned by the United Nations for rocket launches.

The sanctions, aimed at trying to derail the country's rogue nuclear weapons and ballistic missile programs, bar North Korea from testing or using nuclear or ballistic missile technology, and from importing or exporting material for these programs.

The latest sanctions resolution again demanded that North Korea abandon its nuclear weapons program and cease launches. It slapped sanctions on North Korean companies and government agencies, including its space agency and several individuals.