North Korea is close to developing a nuclear warhead small enough to be mounted on a long-range ballistic missile, a Chinese expert said in Seoul Wednesday.

Speaking at a forum hosted by the Asan Institute for Policy Studies in the South Korean capital, Li Bin, a professor at China's Tsinghua University, said North Korea during its last nuclear test in February could not complete the task of miniaturization of a nuclear warhead, the Yonhap News Agency reported.

However, he said, "if they have a chance for more nuclear tests, maybe one more, they would be able to have small and more reliable device for their missile," said Li, who is also a nuclear policy expert at the Carnegie Endowment for International Peace.

Li said during its first test in 2006, North Korea began with a small device with a small amount of explosives, but that was not so successful.

"Then they had to add more chemical explosives because the yield was not good enough. Eventually they got full yield (in the third test), but the device is not small enough," said the expert.

China has been pressing the United States, Russia, Japan and South Korea to restart the Six-Party talks designed to convince North Korea to end its nuclear weapons program. The North is the sixth member of the talks, which have stalled since 2008.

Lately, there have been reports indicating the North may be developing intercontinental ballistic missiles or ICBMs.

Li also said the North may have tested a plutonium-based nuclear weapon, instead of uranium-based one, because it is more difficult to miniaturize bombs using uranium, Yonhap reported.

Joshua Pollack, a nuclear expert at Science Applications International Corp., said the North may be internally producing components for gas centrifuge, used for uranium enrichment, Yonhap said.