South Korea (search) said it was trying to confirm whether North Korea (search) is preparing its first nuclear weapons test and urged the North to return to six-nation disarmament talks to ensure that its future remains bright, while a top U.S. envoy said the talks were in doubt.

North Korea declared in February that it had nuclear weapons and was boycotting the nuclear talks. International experts believe Pyongyang (search) may have at least two or three bombs and recent media reports said it might be preparing for its first nuclear test.

Efforts to get the North back to the talks, which include the two Koreas, China, the United States, Japan and Russia, have floundered since last June.

South Korean Foreign Minister Ban Ki-moon (search) said Wednesday that Seoul is working with other nations to confirm the reports about North Korea's possible nuclear test.

"We once again urge North Korea to quickly return to the six-party talks for the peace and stability on the Korean Peninsula and Northeast Asia, and also for the North's own bright future," Ban told a regular media briefing.

Earlier this week, Ban warned North Korea against conducting a nuclear test amid fears the isolated state is trying to harvest plutonium for more weapons after it apparently shut down a nuclear reactor.

In Beijing on Wednesday, the top U.S. envoy on the North Korean nuclear issue Christopher Hill (search) said that the future of six-nation talks was uncertain but that diplomatic efforts must continue.

Hill, a U.S. assistant secretary of state, said that in meetings Tuesday with Chinese officials, "I made it clear that we do have a lot of options, but one option we don't have is to walk away from this."

Washington is reportedly exploring other options to stop North Korea from building up its alleged nuclear arsenal.

U.S. Secretary of State Condoleezza Rice (search) said this week the United States reserves the right to seek U.N. Security Council action on North Korea but remains committed for the time being to seeking a negotiated end to its nuclear weapons program.

"We still believe there is a lot to be done within the six-party framework," Rice said, alluding to disarmament negotiations that began almost two years ago. There have been no talks since June 2004 because of North Korea's unwillingness to renew the discussions.