North Korea leader Kim Jong-un calls for nuclear arsenal expansion to 'strike at US imperialists at any time'

Remarks follow last week's claims the rogue state had successfully tested a hydrogen bomb

North Korean leader Kim Jong-un (AP)

North Korea’s leader Kim Jong-un called for his country to expand the size and power of its nuclear arsenal so it could make “strikes at US imperialists at any time and in any place, state media said early Wednesday.

A week after the country drew widespread condemnation for a nuclear test Kim "set forth the important tasks to be fulfilled to bolster up the nuclear force,” the official KCNA state news agency said.

Speaking at a ceremony to award scientists and others behind last week's nuclear tests, which North Korea has been touting in state media as a major achievement he said the developments should be made to prevent the US from encroaching “upon the sovereignty of the DPRK and make threatening provocations."

The North's official name is Democratic People's Republic of Korea.

Experts from the US and other countries have expressed doubts about the North’s claims that last week’s test was of a more powerful hydrogen bomb, as the blast was of about the same size as that from an atomic bomb test in 2013.

However, the US House of Representatives voted nearly unanimously to pass legislation that would broaden sanctions over North Korea's nuclear program.

The call came hours after it was revealed that the country had detained a US citizen on suspicion of spying, according to CNN.

A man identified as Kim Dong Chul was being held by the Pyongyang government and that authorities had accused him of engaging in spying and stealing state secrets, the broadcaster said.

Kim said he had traveled extensively in recent years between China and North Korea and had made some trips to South Korea as well.

The state run DPKR news service also said that the country’s foreign Minister Roh Song-Tae had telegrammed “congratulations to Iran on successful capture of US backed pirates in Iranian waters,” after two American naval vessels and 10 troops were detained by the Middle Eastern country on Tuesday.