The White House on Monday blasted the latest moves by the Democratic People's Republic of Korea (DPRK), vowing to defend US allies against the threats.

Spokesman Jay Carney was responding to DPRK's artillery shells fired into South Korean waters in a live-fire drill on Monday, and to its threat to conduct a "new form" of nuclear test in response to continued US hostile policy.

"North Korea's actions are dangerous and they're provocative," Carney said at a regular news briefing. "And the DPRK's continued threats and provocations aggravate tensions and further North Korea's isolation."

"We remain steadfast in our commitment to the defense of our allies," he added. "And any nuclear tests or any ballistic missile launches would be significant violations of U.N. Security Council resolutions."

South Korea's military fired back on the north with its K-9 self-propelled howitzers, and dispatched F-15K fighter jets to the sea border.

Tensions have escalated on the Korean Peninsula after Pyongyang fired a volley of short- and medium-range missiles in recent weeks.

The UN Security Council issued a statement on Friday condemning the DPRK's recent ballistic missile launches, prompting Pyongyang to declare on Sunday that it would not "rule out a new form of nuclear test" to bolster up its nuclear deterrence.

The DPRK conducted three nuclear tests respectively in October 2006, May 2009 and February 2013.

Carney said Washington will maintain "close coordination" with South Korea and Japan.

The State Department said on Monday that Robert King, US special envoy for DPRK human rights issues, will visit the two allies on April 2-10 for regular consultations over "a range of human rights and humanitarian issues."