North Korea has threatened unspecified “offensive action” over the expansion of US military exercises with rival South Korea as President Joe Biden’s special representative for North Korea flew to Seoul for talks with allies over the North’s growing nuclear threat.

The North Korean comments came a day after the United States flew nuclear-capable B-52 bombers to the Korean Peninsula for joint aerial exercises with South Korean warplanes in their latest show of force against the North, which portrays the allies’ drills as invasion rehearsals.

Animosity heightened in recent weeks as the pace of both the US-South Korean military exercises and the North Korean weapons demonstrations increased in a cycle of tit-for-tat.

Sung Kim, the US special representative to North Korea, arrived in Seoul for talks with South Korean and Japanese officials to coordinate their response to North Korea’s intensifying weapons development and threats of nuclear conflict.

Following meetings with South Korean Foreign Minister Park Jin and other South Korean officials on Thursday, Kim will take part in a three-way meeting with the South Korean and Japanese nuclear envoys on Friday, according to South Korea’s Foreign Ministry.

READ MORE:S Korea, US and Japan hold submarine drills amid tension with Pyongyang

'Irreversible catastrophe'

North Korea on Thursday accused the US and South Korea of escalating tensions to the brink of nuclear war through their joint military drills, vowing to respond with "offensive action," the North’s official Korean Central News Agency reported.

KCNA released a commentary by Choe Ju Hyon, whom it called an international security analyst, criticising the exercises as "a trigger for driving the situation on the Korean peninsula to the point of explosion."

"The reckless military confrontational hysteria of the US and its followers against the DPRK is driving the situation on the Korean peninsula to an irreversible catastrophe ... to the brink of a nuclear war," the article said.

It was using the acronym of North Korea's official name, the Democratic People's Republic of Korea.

"Now the international community unanimously hopes that the dark clouds of a nuclear war hanging over the Korean peninsula will be removed as early as possible," it added.

Denuclearisation talks

Sung Kim on Thursday separately met with South Korean Foreign Minister Park Jin and chief South Korean nuclear negotiator, Kim Gunn, where they discussed strengthening joint defence postures and inducing further international efforts to crack down on illicit North Korean activities funding its weapons program, the South's foreign ministry said.

They discussed seeking an active role from China – North Korea’s key ally and economic lifeline – in persuading Pyongyang to halt its weapons displays and return to denuclearisation talks, according to the South Korean Foreign Ministry. 

Beijing and Moscow have blocked US-led attempts to strengthen UN sanctions against the North over some of its ballistic tests, underscoring a divide in the Security Council deepened over Russia’s offensive on Ukraine.

Sung Kim and Kim Gunn are planning to participate in a three-way meeting with Japanese nuclear envoy Takehiro Funakoshi in Seoul on Friday.

North Korea in March alone fired nearly 20 missiles over seven different launch events, including an intercontinental ballistic missile that demonstrated potential range to reach the US mainland and several shorter-range weapons designed to deliver nuclear strikes on South Korean targets.

Tensions are likely to prolong as North Korea is likely to use the allies’ continuing drills as a pretext to advance weapons development and intensify military training involving its nuclear-capable missiles.

READ MORE: Is the threat of North Korea spurring Japan-South Korea rapprochement?