North Korea on Tuesday vowed "merciless" retaliation against the South for its support of UN sanctions, as Seoul urged Pyongyang to step back from a widely expected nuclear test.

The perennially tense situation on the Korean peninsula has been stretched to its limit in the past week, with almost daily threats from the North that it is preparing to conduct a nuclear test as a riposte to the expanded sanctions.

A lengthy commentary published Tuesday by the official Korean Central News Agency (KCNA) reiterated that the sanctions resolution passed by the UN Security Council was tantamount to a "declaration of war".

Noting what it called the South's "despicable" support for the resolution, KCNA said it was an act of gross provocation that would not go unanswered.

"The provokers will meet only merciless retaliatory blows," it said.

The commentary made no specific mention of the nuclear test that the North's top military body had explicitly signalled in a statement carried by KCNA last week.

The National Defence Commission had said the test was aimed at "arch-enemy" the United States, which had proposed the UN resolution penalising Pyongyang for a banned rocket launch in December.

In Seoul, the foreign ministry on Tuesday noted that the UN resolution had warned of further "significant action" against the North it it proceeds with another test.

"The government once again strongly urges North Korea to pay heed to the continued warnings from the international community and not push ahead with any further provocations, including a nuclear test," spokesman Cho Tai-Young said.

"I don't really understand why North Korea is sticking to an act that threatens security in the region at a time when its people are struggling from a lack of food," Cho said.

South Korean defence officials have said they believe the North is capable of conducting a nuclear test "any time" and announced Tuesday the creation of a special task force to monitor the site of its two previous tests in 2006 and 2009.

The task force was set up by the South's Joint Chiefs of Staff and is studying data and intelligence being collected by state institutions and independent experts, defence ministry spokesman Kim Min-Seok told reporters.

The state-run Korea Institute of Geoscience and Mineral Resources has set up a situation room to receive and process seismic information that might indicate when a test is conducted, he added.

The institute detected an "artificial earthquake" about 40 seconds after the North's second nuclear test in 2009.