North Korea's Korean Central News Agency released photographs of missiles it launched last week in response to joint military drills carried out by the US and South Korea, dubbed Vigilant Storm, on Monday. (Yonhap)

South Korea, the United States and Japan on Monday strongly condemned North Korea for launching an unprecedented number of ballistic missiles last week, and warned that such provocative actions would only further isolate the regime.

In a trilateral talk over the phone, South Korean First Vice Foreign Minister Cho Hyun-dong spoke with his American and Japanese counterparts, Wendy Sherman and Takeo Mori, to discuss cooperation measures to handle North Korea’s missile threats, Seoul’s Foreign Ministry said.

The three officials pointed out how Pyongyang's latest missile tests clearly violate multiple United Nations Security Council resolutions and seriously threaten the overall peace and stability of international society.

They also reaffirmed their commitments to work closely together to make sure a “stern,” and “unified” message can be formed and delivered to Pyongyang in upcoming multilateral summits, including the East Asia Summit and the G20 Summit.

The three sides also assessed their frequent and close communication as positive, and agreed to continue discussions on their common interests, at all levels, the ministry said.

Revealing in detail the military operations it conducted last week, North Korea vowed to respond "more mercilessly," against the joint military activities carried out by the US and South Korea, Monday.

On Monday, North Korea revealed in detail of “military operations” it conducted last week in response to joint military drills the US and South Korea carried out, dubbed Vigilant Storm. Against the two joint military exercises, the North said it would respond “more mercilessly.”

Pyongyang’s state-operated Korean Central News Agency reported the General Staff of the Korean People’s Army conducted a four-day military operation from Wednesday, as to respond to Seoul and Washington's joint military exercise Vigilant Storm.

In the report, it also said it fired strategic cruise missiles just 80 kilometers off the southeastern coast of Ulsan.

"The recent corresponding military operations by the KPA are a clear answer of the DPRK that the more persistently the enemies' provocative military moves continue, the more thoroughly and mercilessly the KPA will counter them," the KPA said in the English-language statement. The KPA referred to North Korea by the acronym of its official name, Democratic People’s Republic of Korea.

On Pyongyang's test-firing of the strategic cruise missiles, Seoul's Joint Chiefs of Staff said it may be a false claim as it could not find the evidence in its joint analysis with the US intelligence authorities.

On Monday, Seoul kicked off its annual computer-simulated Taegeuk training to improve its operational capabilities against North Korean threats.

Last week, North Korea launched more than two dozens of ballistic missiles, which included one intercontinental ballistic missile that the South’s military presume to have failed in-flight.

Pyongyang also launched a short-range ballistic missile that crossed over the de facto maritime military border with Seoul to land close to South Korea’s territorial waters. It was the first time the North launched a missile over the Northern Limit Line since the two Koreas separated at the end of the 1950-53 Korean War.

Aside from missile launches, the North also fired more than 100 rounds of artillery, and also staged a large scale combat sorties involving some 500 fighters.

By Jo He-rim (herim@heraldcorp.com)