North Korea's nuclear program has made strides in recent months but the country has not yet demonstrated all the components of an intercontinental ballistic missile (ICBM), including a survivable re-entry vehicle, the vice chairman of the US Joint Chiefs of Staff said on Tuesday.

Air Force General Paul Selva's remarks confirmed an assessment by Defense Secretary Jim Mattis in December that North Korea's ICBM did not pose an imminent threat to the United States.

"What he has not demonstrated yet are the fusing and targeting technologies and survivable re-entry vehicle," Selva said, referring to North Korean leader Kim Jong Un.

"It is possible he has them, so we have to place the bet that he might have them, but he hasn't demonstrated them," Selva, the second highest-ranking US military official, added.

In November, North Korea said it had successfully tested a new type of ICBM that could reach all of the US mainland and South Korea.

Selva said that if conflict were to break out, it was unlikely the United States would be able to get an early indication of North Korean launches.

"It is very unlikely that in a tactical situation, we would get any of the indications and warning that would precede a launch other than if we got lucky and saw the movement of the launch mechanism to the launch platform," Selva said.

He said that by using mobile erected launchers, the warning time for the United States had decreased from up to an hour to about a dozen minutes.

Meanwhile, a group of South Korean skiers flew to North Korea yesterday ahead of the Winter Olympics to train with their rivals, on a rare direct flight, reported AFP.

The 45-strong delegation, including 24 skiers travelled to the North's Masikryong ski resort, a pet project of leader Kim Jong-Un.