North Korea could soon develop long-range missiles capable of striking the United States, the top US commander in the Pacific has warned.

Admiral Harry Harris told the House Armed Services Committee he believed Pyongyang's threats against the US needed to be taken seriously.

"Just as Thomas Edison is believed to have failed 1,000 times before successfully inventing the light bulb, so, too, Kim Jong-un will keep trying," he said.

"One of these days soon, he will succeed."

US strike on North Korea 'would have cataclysmic consequences'

North Korea's growing nuclear and missile threat is perhaps the most serious security challenge facing President Donald Trump.

Admiral Harris said the Terminal High Altitude Area Defence (THAAD) system would be made operational within days, and said any North Korean missile fired at US forces would be destroyed.

"If it flies, it will die," he told the committee.

He warned that while the defences of Hawaii were sufficient for now, they could one day be overwhelmed.

"I don't share your confidence that North Korea is not going to attack either South Korea, or Japan, or the United States... once they have the capability," the Admiral said.

In pictures: North Korea military drill Show all 8 1 /8 In pictures: North Korea military drill In pictures: North Korea military drill North Korea's leader Kim Jong Un watches a military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) KCNA/Handout via REUTERS In pictures: North Korea military drill A military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) is seen in this handout photo by North Korea's Korean Central News Agency (KCNA) KCNA/Handout via REUTERS In pictures: North Korea military drill A military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) is seen in this handout photo by North Korea's Korean Central News Agency (KCNA) KCNA/Handout via REUTERS In pictures: North Korea military drill A military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) KCNA/Handout via REUTERS In pictures: North Korea military drill A military drill marking the 85th anniversary of the establishment of the Korean People's Army (KPA) KCNA/Handout via REUTERS In pictures: North Korea military drill This image made from video of still images broadcast in a news bulletin by North Korea's KRT, shows what was said to be a 'Combined Fire Demonstration' held to celebrate the 85th anniversary of the North Korean army, in Wonsan, North Korea. KRT via AP Video In pictures: North Korea military drill This image made from video of still images broadcast in a news bulletin by North Korea's KRT, shows what was said to be a 'Combined Fire Demonstration' held to celebrate the 85th anniversary of the North Korean army, in Wonsan, North Korea. KRT via AP Video In pictures: North Korea military drill This image made from video of still images broadcast in a news bulletin by North Korea's KRT, shows what was said to be a 'Combined Fire Demonstration' held to celebrate the 85th anniversary of the North Korean army, in Wonsan, North Korea. KRT via AP Video

Washington and Pyongyang have stepped up warnings to each other in recent weeks amid concerns Pyongyang may soon conduct a sixth nuclear bomb test.

Washington has said all options are on the table, including military strikes, but officials have stressed the current focus is on tightened sanctions on North Korea, which are expected to be discussed at a UN Security Council meeting on Friday chaired by US Secretary of State Rex Tillerson.

While North Korea has yet to test a missile capable of reaching the United States, experts say it could have the capability some time after 2020

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US officials have warned a conflict with North Korea could have a devastating effect on ally South Korea and US troops based there, a point Pyongyang underscored by its largest ever artillery drill to mark the foundation of its military.

Admiral Harris conceded that North Korean retaliation to any US strikes could cause many casualties in South Korea, but added that there was the risk "of a lot more Koreans and Japanese and Americans dying if North Korea achieves its nuclear aims and does what [North Korean leader Kim Jong-un] has said it's going to do."

North Korea has vowed to strike the United States and its Asian allies at the first sign of any attack on its territory.