At present it would be “lucky to hit even a city-sized target”, he said, citing limits to its re-entry technology.

But with “a year or two of additional testing and development”, he added, “it will likely become a missile that can reliably deliver a single nuclear warhead to targets along the US west coast, possibly with enough accuracy to destroy soft military targets like naval bases”, such as that at San Diego in California.

The North’s missile technology – which it is banned from developing by the UN Security Council – has advanced rapidly under Kim, ramping up tensions between Pyongyang and Washington. The impoverished state has also staged five nuclear tests – including two last year.

Washington is to propose tougher UN sanctions against the North, but analysts say they will have a limited impact unless China – the North’s sole major ally and economic lifeline – steps up pressure on its neighbour.

Beijing is reluctant to risk destabilising the North, fearing a potential influx of refugees along the frontier or US troops stationed on its border in a unified Korea.

Meanwhile, South Korea on Tuesday disputed North Korea’s claim to have an ICBM that can strike the US mainland, saying Kim’s regime may not yet have re-entry capability for such projectiles.

The re-entry feature of North Korea’s missile programme is not established, said Yi Wan-young, a South Korean lawmaker who was briefed by National Intelligence Service chief Suh Hoon on Tuesday in a closed-door session for lawmakers. Re-entry capability is critical for intermediate range missiles and ICBMs.

A nuclear warhead needs to be housed in a vehicle that can survive the heat of returning to the Earth’s atmosphere, Jeffrey Lewis, director of the East Asia Non-Proliferation Programme at the Middlebury Institute of International Studies at Monterey, California, wrote on the 38 North website.

South Korea’s new Foreign Minister Kang Kyung-wha told lawmakers at a parliamentary session on Monday that Seoul is discussing the option of secondary American sanctions on entities and individuals that trade with North Korea.

NIS chief Suh also said on Tuesday that North Korea is on standby to conduct a sixth nuclear test, but this doesn’t appear to be imminent, according to the lawmaker.

Kim Jong-un, for his part, was feted at a concert replete with pop music and thunderous applause marking last week’s successful launch of his country’s first intercontinental ballistic missile.

Among the numbers performed were Song of Hwasong Rocket and Make Others Envy Us, the North’s official KCNA reported.

Leading the bill at the concert was the Moranbong Band, an all-female ensemble that was hand-picked by Kim and serves as something of the “soft” face of his regime.

Pyongyang has been the scene of a series of special events to mark the July 4 launch of Hwasong-14, which is believed to be capable of reaching most of Alaska and possibly farther.

The launch has been treated as a major national milestone in North Korea, with the government and media touting it as a technological breakthrough few other nations have achieved.

The concert, which took place on Sunday before a packed crowd, many in uniform, also featured dancing. Clips of the concert shown on North Korean television showed the crowd repeatedly cheering and applauding for Kim.

Along with the Moranbong Band, the concert had North Korea’s other top headliners – a similar pop ensemble called the Chongbong Band, the uniformed State Merited Chorus and the Wangjaesan Art Troupe.

Additional reporting by Bloomberg, Associated Press