China’s top nuclear experts have increased their estimates of North Korea’s nuclear weapons production well beyond most previous U.S. figures, suggesting Pyongyang can make enough warheads to threaten regional security for the U.S. and its allies.

The latest Chinese estimates, relayed in a closed-door meeting with U.S. nuclear specialists, showed that North Korea may already have 20 warheads, as well as the capability of producing enough weapons-grade uranium to double its arsenal by next year, according to people briefed...