China's nuclear experts have warned that North Korea may already have as many as 20 nuclear warheads and can produce weapons-grade uranium to double its nuclear arsenal by next year, the Wall Street Journal reported Thursday.

The Chinese estimate of North Korea's nuclear capability, which was conveyed to U.S. nuclear scientists during a closed-door meeting in February, is above previous U.S. estimates of the North's nuclear arsenal.

The report cited people who were briefed on the issue.

North Korea, which unveiled its uranium enrichment facility in 2010, has conducted three nuclear tests so far.

In February, experts at the U.S.-Korea Institute at Johns Hopkins University said that North Korea is currently believed to have 10 to 16 nuclear weapons, with six to eight of them based on plutonium and four to eight based on weapons-grade uranium.

The U.S. experts said Pyongyang's nuclear stockpile could expand to as many as 100 weapons by 2020. (Yonhap)