Chinese nuclear experts have warned that North Korea may already have 20 nuclear warheads and the capability to produce enough weapons-grade uranium to double its arsenal by next year, The Wall Street Journal has reported.

The Chinese estimates of Pyongyang's nuclear production, relayed to US nuclear specialists, exceed most previous US forecasts, which range from 10 to 16 bombs currently, said the report, which cited people briefed on the matter.

Experts at the US-Korea Institute at Johns Hopkins University's School of Advanced International Studies earlier this year presented three scenarios for North Korea's nuclear capability, predicting its stockpile of weapons could grow to 20, 50 or 100 within five years.

North Korea has conducted three nuclear detonations, the most recent in February 2013.

Early this month, US Admiral William Gortney, commander of the US Northern Command and North American Aerospace Defense Command, said the US military believes North Korea has the ability to miniaturise a warhead and mount it on a ballistic missile, although there had been no tests.