SEOUL (Reuters) - 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 reported.

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

Experts at the U.S.-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.

Asked about the report at a regular press briefing on Thursday, China's Foreign Ministry spokesman Hong Lei said he "did not have knowledge of the specific situation".

"We must persevere with denuclearization on the Korean peninsula, persevere in safeguarding peace and stability, and persevere in resolving the relevant issue through dialogue," Hong said.

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

(Reporting by Tony Munroe in Seoul and Adam Rose in Beijng; Editing by Paul Tait and Simon Cameron-Moore)