South Korean officials say they are trying to determine the cause of a huge explosion Thursday reported in North Korea near a missile base of the communist state.

South Korea's Yonhap news agency quoted South Korean presidential office spokesman Kim Jong-min as saying officials did not think it was a nuclear test.

South Korean Unification Minister Chung Dong-young told reporters in Seoul Sunday he doubts the blast was a North Korean nuclear weapons test.

South Korean media, quoting diplomats in Seoul, say Thursday's explosion sent up a mushroom cloud with a radius of at least 3.5 kilometers, rising not far from the Chinese border where North Korea has a base for its Daepodong missiles.

News of the explosion comes amid intense speculation in Washington and Seoul that North Korea might be preparing for its first nuclear weapons test.

But diplomatic sources in Tokyo on Sunday cautioned against concluding the blast was deliberate or linked to the country's nuclear program. They speculate, based on preliminary descriptions, that it sounds as if the explosion could have been caused by rocket fuel.

This comes as China, South Korea, Japan and the United States are holding various meetings on how to press North Korea into a fourth round of negotiations on dismantling its nuclear programs.