North Korean leader Kim Jong Un inspects nuclear warheads at an undisclosed location
A screen grab shows North Korean leader Kim Jong Un inspecting nuclear warheads at an undisclosed location in this undated still image used in a video. KRT/via Reuters TV/Handout via Reuters

North Korean leader Kim Jong Un called for strengthening the country's war deterrence in a "more practical and offensive" manner to counter what it called moves of aggression by the U.S. and South Korea, state media KCNA said on Tuesday.

Kim made the comment at an enlarged meeting of the Central Military Commission held on Monday to discuss ongoing efforts to boost the country's war deterrent to "cope with the escalating moves of the U.S. imperialists and the south Korean puppet traitors to unleash a war of aggression," KCNA said.

North Korea has been reacting angrily to a recent series of the allies' joint military exercises, which KCNA said were intended to prepare for "an all-out war" and forced Pyongyang to explore "powerful practical action", including military options.

Kim ordered further reinforcing the country's war deterrence with "increasing speed" and in a "more practical and offensive" manner, KCNA said.

The meeting "discussed practical matters and measures for machinery to prepare various military action proposals that no means and ways of counteraction are available to the enemy," it added.

South Korea and the U.S. forces have conducted annual springtime exercises since March, including air and sea drills involving a U.S. aircraft carrier and B-1B and B-52 bombers, and their first large-scale amphibious landing drills in five years.

North Korea has also carried out various military activities in recent weeks, unveiling new, smaller nuclear warheads, testing what it called a nuclear-capable underwater attack drone and firing an intercontinental ballistic missile capable of striking anywhere in the United States.

Seoul's unification minister Kwon Young-se, in charge of handling cross-border affairs, expressed strong regret on Tuesday over Pyongyang not responding on inter-Korean hotlines since last week, calling it "irresponsible."

The two Koreas operate border hotlines to prevent unexpected incidents, but the North has often refused to receive messages or daily check-in calls from the South when relations are strained.

"We express strong regret over North Korea's unilateral and irresponsible attitude, and strongly warn that it will eventually isolate itself and put itself in more difficult circumstances," Kwon told a news conference.

Kwon also denounced the North for using South Korean assets left at a joint factory park, which the North unilaterally shut in 2016, without approval, pledging to explore measures to hold it accountable.

Senior defence officials from South Korea and the U.S. were set to hold their annual Korea-U.S. Integrated Defense Dialogue (KIDD) in Washington on Monday and Tuesday, Seoul's defence ministry said.

Topics of discussion included joint responses to the North's nuclear and missile threats and ways of improving the American nuclear umbrella's execution.

Japan will then join trilateral defence talks on Friday. The three countries last week expressed concern over North Korea using "malicious" cyber activities to bankroll its weapons programmes.

Tokyo's chief cabinet secretary Hirokazu Matsuno told reporters on Monday that Pyongyang could stage further provocations, including nuclear tests.