South Korea's military said they had detected the launch of "two short-range ballistic missiles fired from Sukchon areas in South Pyongan province between 0700-0711 (2200-2211 GMT)".

Tokyo also confirmed the launch, with the Prime Minister's Office warning the North had launched "a suspected ballistic missile" and the coastguard issuing alerts over multiple projectiles.

North Korea issued a statement soon after saying it had "fired two shots using 600mm multiple rocket launchers", into the East Sea, referring to the body of water also known as the Sea of Japan.

The launch comes less than 48 hours after Pyongyang conducted what it called a "surprise" drill to fire an intercontinental ballistic missile Saturday, which it said demonstrated North Korea's capacity to carry out a "fatal nuclear counterattack".

Japan said Saturday's ICBM had flown for 66 minutes and landed in its exclusive economic zone (EEZ). In response, Seoul and Washington staged joint air drills Sunday, featuring a strategic bomber and stealth fighter jets.

In a statement issued early Monday, North Korean leader Kim's sister Kim Yo Jong warned that Pyongyang would continue to take "corresponding counteraction" to any perceived threats.

"The frequency of using the Pacific as our firing range depends upon the US forces' action character," she said in a statement carried by the official KCNA, which also strongly critiqued outside assessments of the Saturday ICBM launch.

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

Kim Yo Jong dismissed such criticism as "a bid to undervalue the preparedness of the DPRK missile forces", she said, referring to the North by its official name.

- Angry response -

Hong Min of the Korea Institute for National Unification told AFP that the angry reaction was part of a "pattern" of North Korea pushing back against any outside assessment of their ICBM capacities.

"Kim's strong and angry reaction on outside assessment of its ICBM launch show the North really cares about delivering a message that it is capable of hitting the US mainland," he said.

The Monday launch was North Korea's "response to Korea-US joint aerial exercise over the weekend", he said.

Using shorter range missiles indicated North Korea was "virtually targeting US bases and South Korean command centre in the area".

The sanctions-busting North Korean launches came just before Seoul and Washington are due to start joint tabletop exercises later this week aimed at improving their response in the event of a North Korean nuclear attack.

Pyongyang warned last week of an "unprecedentedly" strong response to upcoming drills, which it describes as preparations for war and blames for the deteriorating security situation on the Korean peninsula.

Relations between the two Koreas are at one of their lowest points in years, after North Korea declared itself an "irreversible" nuclear state and leader Kim called for an "exponential" increase in weapons production, including tactical nuclear weapons.