Seoul, Sep 26 (IANS/EFE): North Korea may need only one more nuclear test before developing a nuclear warhead small enough to mount on a long-range ballistic missile, a Chinese expert said Wednesday.

"If Pyongyang can conduct more tests, maybe with just one more it would be able to have a smaller, more reliable device for its missiles," nuclear policy expert Li Bin, a professor at China's Tsinghua University, said at a symposium on North Korea hosted by the Asan Institute for Policy Studies here.

In North Korea's first nuclear test in 2006, it used a small device with a small amount of explosives that was not successful, according to Li, a member of the Chinese delegation that has sought a negotiated end to Pyongyang's nuclear weapons programme.

"Then they had to add more chemical explosives because the yield wasn't good. In the third test, they got full yield but the device wasn't small enough," Li said.

The Chinese expert also said North Korea may have used plutonium in its latest test in February as opposed to uranium, since it is more difficult to miniaturise warheads using the latter.

Since then, China has tried to force North Korea to return to the so-called Six Party Talks - involving the US, China, Russia, Japan and the two Koreas - aimed at ending the isolated country's nuclear programme.

Most experts believe Pyongyang still has not developed a nuclear warhead small enough to mount on a ballistic missile.