N Korea preparing for fourth nuclear test: South

Intelligence reports indicate heightened activity at N Korea's main atomic test site

North Korea appears to be preparing for a fourth nuclear test, South Korea today said, following intelligence reports of heightened activity at its main atomic test site.

"There are such signs," Unification Minister Ryoo Kihl-Jae told a parliamentary committee hearing when lawmakers asked him to confirm the reports.

It was the North's third nuclear test in February and subsequent UN sanctions that kickstarted the cycle of ongoing escalating military tensions that the international community is desperate to break.

The JoongAng Ilbo daily said South Korean intelligence had detected unusually busy personnel and vehicle movements at the North's Punggye-ri nuclear test site since last week.

"We are trying to figure out whether it is a genuine preparation for a nuclear test or just a ploy to heap more pressure on us and the US," it cited an unnamed South Korean government official as saying.

Intelligence reports also suggest Pyongyang has readied two mid-range missiles on mobile launchers on its east coast, and is aiming at a test firing before the April 15 birthday of late founding leader Kim Il-Sung.

A missile launch would be highly provocative, especially given the strong rebuke the North's sole ally China handed it on the weekend and a US concession to delay its own planned missile test.

Kim Jang-Soo, chief national security adviser to President Park Geun-Hye, said a test-launch could come before or after Wednesday, the date by which the North has suggested foreign diplomats consider leaving Pyongyang.

Japan has ordered its armed forces to shoot down any North Korean missile headed towards its territory, a defence ministry spokesman in Tokyo said Monday.

The New York Times reported that the US and South Korea have drawn up plans for a measured tit-for-tat response to North Korean actions, which will be limited in order to prevent an escalation to broader war.

Citing unnamed US officials, the newspaper said the new "counter-provocation" strategy calls for an immediate but proportional "response in kind" if the North decides to launch a missile or ground attack.

North Korea's bellicose rhetoric has reached fever pitch in recent weeks, with near-daily threats of attacks on US military bases and South Korea in response to ongoing South Korea-US military exercises.

There has been growing global concern that the crisis could spiral out of control, and China appeared to respond at the weekend to repeated calls for it to take a harder line with Pyongyang.