HONG KONG — North Korea said it has developed the technology to mount a nuclear warhead on a missile, signaling the country may now have the ability to launch a nuclear attack against the United States.

South Korea and the U.S. military have been divided over whether North Korea could shrink a warhead sufficiently to fit it on a nuclear-tipped missile. A spokesman at the National Defense Commission in Pyongyang said the military has mastered the engineering and will diversify its nuclear weapons, the official Korean Central News Agency reported Wednesday.

North Korea has successfully detonated three nuclear devices at a test site and has been improving the range of its ballistic missiles in defiance of United Nations sanctions over its weapons program. The U.S., China and South Korea have been unable to convince the Kim Jong Un regime to return to disarmament talks, and there are signs that North Korea is rapidly expanding its nuclear arsenal.

The announcement comes weeks after the country released video of Kim watching what the official media said was the launch of a ballistic missile from a submarine. U.S. and South Korean officials have questioned the veracity of the test, with Defense Ministry spokesman Kim Min Seok saying on May 11 that the North was still likely years away from being able to master a submarine missile launch.

South Korea has also questioned whether its rival can miniaturize a warhead, with Kim Min Seok saying in February there was no evidence that the government in Pyongyang has the ability to tip a missile with a nuclear warhead.

That assessment contrasts with the position of William Gortney, the head of the U.S. Northern Command, who said April 7 that North Korea does have the technology and has also managed to deploy a road-mobile intercontinental ballistic missile — the KN-08 — capable of reaching the U.S.

“Our assessment is that they have the ability to put a nuclear weapon on a KN-08 and shoot it at the homeland,” Gortney said. “We have not seen them do that” and “we haven’t seen them test the KN-08.”

North Korea last tested a nuclear device in February 2013; just how many warheads North Korea has been able to build remains a mystery. Top Chinese nuclear weapons experts have increased their estimates of North Korean warhead production beyond most previous U.S. projections, the Wall Street Journal reported on April 23.

The Kim regime has 20 warheads and has the capacity to produce enough weapons-grade uranium to double its arsenal by next year, the paper reported, citing people briefed on the matter. That compares with the estimate of 10 to 16 warheads released in February by U.S. researcher Joel Wit.

With assistance from Seyoon Kim in Seoul.