North Korea could produce enough plutonium for a nuclear weapon from a reactor it restarted in September 2015, the director of US National Intelligence said on Tuesday.

Intelligence chief James Clapper also said Pyongyang has improved the sophistication of its ballistic missile programme.

North Korea conducted a long-range rocket launch on Sunday, following a nuclear test in January, throwing the security balance in the region into disarray.

"We assess that North Korea has followed through on its announcement by expanding its Yongbyon enrichment facility and restarting the plutonium production reactor," Clapper said in his annual report on the threats the US faces.

Clapper also said Pyongyang was "committed to developing a long-range, nuclear-armed missile that is capable of posing a direct threat to the United States."

North Korea had halted operations at its Yongbyon reactor in 2008 following a 'disarmament-for-aid' deal.

However, after the UN slapped sanctions on it after a third nuclear test in 2013, Pyongyang said it was refurnishing the reactor.

The US experts now believe the reactor is now capable of producing enough plutonium for a nuclear weapon.

"We further assess that North Korea has been operating the reactor long enough so that it could begin to recover plutonium from the reactor's spent fuel within a matter of weeks to months," Clapper said in his report.

According to the BBC, when the reactor is fully operational, it could produce about 4kg of plutonium every year, which is enough to make one bomb.

In September last year, Pyongyang had claimed Yongbyon was operational and that development of a weapon it could use against the US "at any time" was progressing.

Amid wide condemnation, North said its rocket launch on Sunday was intended to put an Earth satellite on orbit, but the US, South Korea and Japan believe it was part of a secretive programme to develop a missile system that can carry nuclear warheads in future.

Following the rocket launch, the US and South Korea started talks on deploying the sophisticated Terminal High Altitude Area Defense (THAD) system to forestall any threat from Pyongyang.