South Korea said Thursday it had staged joint air drills with the United States featuring strategic bombers and stealth fighters, prompting Pyongyang to warn that such exercises could "ignite an all-out showdown".

The exercises, the first by the security allies this year, came a day after US Defense Secretary Lloyd Austin and his South Korean counterpart vowed to boost security cooperation to counter an increasingly belligerent nuclear-armed North Korea.

The drills on Wednesday showed "the US's will and capabilities to provide strong and credible extended deterrence against North Korea's nuclear and missile threats," the South Korean defence ministry said.

They involved American B-1B long-range heavy bombers and stealth fighters -- US Air Force F-22s and South Korean F-35s -- flying over the Yellow Sea, the ministry added.

A North Korean foreign ministry spokesman warned the exercises could "ignite an all-out showdown", the state news agency KCNA reported.

Seoul and Washington's moves to ramp up joint drills crossed "an extreme red-line".

South Korea is eager to convince its increasingly nervous public of America's robust defence commitment, after a year in which North Korea declared itself an "irreversible" nuclear power and conducted a weapons test almost every month in defiance of international sanctions.

Austin and South Korean Defence Minister Lee Jong-sup agreed this week to "expand and bolster the level and scale" of joint military exercises in light of "continued provocations" from Pyongyang, including a recent drone incursion into the South.

- 'Nuke for nuke' -
Bolstering US-South Korean military drills and deploying strategic weapons to the region was akin to "talking about the use of nuclear weapons against the DPRK", the North Korean statement on KCNA said, using the country's official name.

It warned that North Korea would follow the "principle of 'nuke for nuke and an all-out confrontation for an all-out confrontation!'"

"The DPRK is not interested in any contact or dialogue with the U.S. as long as it pursues its hostile policy and confrontational line," it added.

US-South Korean joint military exercises infuriate Pyongyang, which views them as rehearsals for an invasion and has often responded with threats and drills of its own.

"By emphasising that the United States is entirely responsible for the deterioration of the situation on the Korean peninsula, (North Korea) is accumulating legitimacy for the development of its missile and nuclear weapons programmes," said Yang Moo-jin, a professor at the University of North Korean Studies in Seoul.

He added that a large North Korean military parade and its planned launch of a spy satellite could further raise tensions with Seoul and Washington.

Commercial satellite imagery has suggested that "extensive parade preparations" are under way in Pyongyang ahead of one of the biggest state holidays, according to the 38 North website.

The parade could be held on the "Day of the Shining Star" on February 16, the birthday of Kim Jong Il, the son of North Korea's founder Kim Il Sung and the father and predecessor of current leader Kim Jong Un, it added.