Pyongyang had already tested one of its most powerful intercontinental ballistic missiles (ICBMs) on Saturday and warned more was to come, with leader Kim Jong Un's powerful sister calling the Pacific the North's "firing range".

Seoul and Washington, who are moving to bolster security cooperation in the region to address Pyongyang's growing threats, staged joint air drills after the ICBM launch -- further enraging the North, which views such exercises as rehearsals for invasion.

State media outlet KCNA said Monday's missile drill involving "super-large multiple rocket launchers, the tactical nuclear attack means", showed the North's army could deter and counter any US-South Korean exercises, adding the weapons could "reduce to ashes the enemy's operational airfield".

Kim's sister, Kim Yo Jong, said Pyongyang was closely monitoring Washington and Seoul's moves to deploy more US strategic assets to the region, vowing "corresponding counteraction" if needed.

"The frequency of using the Pacific as our firing range depends upon the US forces' action character," she said in a statement published by KCNA.

Relations between the two Koreas are at one of their lowest points in decades, after the North last year declared itself an "irreversible" nuclear power and Kim Jong Un called for an "exponential" increase in weapons production, including tactical nukes.

South Korea's military, which said it had detected the launch of two short-range ballistic missiles early Monday, called the string of weapons tests by Pyongyang "a serious provocation that undermines peace and stability on the Korean peninsula".

And Seoul's foreign ministry rapidly imposed fresh sanctions on four individuals and five entities linked to North Korea's weapons programmes Monday.

"Our government has made it clear that North Korea's provocations will inevitably come at a price," it said in a statement.

The UN Security Council will hold a meeting to discuss the situation on Tuesday.

- 'Surprise' drill -

Pyongyang said its Saturday ICBM launch was a "surprise" drill that demonstrated North Korea's capacity to carry out a "fatal nuclear counterattack".

Such claims intend to demonstrate, in the face of international scepticism, "not only the development of strategic and tactical nuclear forces but also the operational capability to use them", said Leif-Eric Easley, a professor at Ewha University in Seoul

North Korea gave its soldiers an "excellent mark" for carrying out the "sudden launching drill" on Saturday, but South Korean analysts pointed out that the estimated nine hours between the order and the launch was not particularly rapid.

Kim Yo Jong dismissed such criticism on Monday as "a bid to undervalue the preparedness of the DPRK missile forces", she said, using North Korea's official name.

Hong Min of the Korea Institute for National Unification told AFP that the strong reaction was part of a pattern of North Korea pushing back against any external analysis of its ICBM capabilities.

The angry retort "shows the North really cares about delivering a message that it is capable of hitting the US mainland", he said.

The North Korean weapons launches came ahead of a joint US-South Korean tabletop exercise this week aimed at improving their response in the event of a nuclear attack by Pyongyang.

And North Korea warned last week of an "unprecedentedly" strong response to ramped-up drills by Seoul and Washington.

Go Myong-hyun, a researcher at the Asan Institute for Policy Studies, said the tests and rhetoric were all "strategic signalling that North Korea wants to send to the United States that North Korea is now a nuclear and missile power".