Pyongyang has opened the year with a flurry of weapons tests, including what state media has claimed are nuclear-capable underwater drones and the launch of two intercontinental ballistic missiles.

On Monday, Kim attended a meeting of the Central Military Commission to discuss ways to "cope with the escalating moves of the US imperialists and the south Korean puppet traitors to unleash a war of aggression," Pyongyang's Korean Central News Agency said.

Kim ordered that the country's deterrence capabilities be strengthened with "increasing speed" and in a "more practical and offensive" manner.

North Korea last year declared itself an "irreversible" nuclear power, effectively ending the possibility of denuclearisation talks.

Earlier this year, Kim ordered the military to intensify drills to prepare for a "real war".

In response, Washington and Seoul have intensified defence cooperation, staging joint military exercises with advanced stealth jets and high-profile US strategic assets.

North Korea views such exercises as rehearsals for invasion, and on Tuesday described them as "frantic" drills "simulating an all-out war against" Pyongyang.

South Korea on Tuesday also accused North Korea of being "irresponsible" after Pyongyang cut hotline contact with Seoul last week.

For five days, North Korea has not answered the twice-daily calls made through a military hotline and an inter-Korean liaison channel, South Korean Unification Minister Kwon Young-se told reporters.

The links were cut a day after Seoul accused Pyongyang of continued unauthorised use of a joint industrial complex in the North Korean city of Kaesong.

South Korea said North Korea had refused to accept its notice to cease activity at the factory, which was once a symbol of reconciliation.

Seoul pulled out of the venture in 2016 following a North Korean nuclear test, saying the complex was helping to fund Pyongyang's weapons programmes.