North Korean leader Kim Jong-un has threatened to "wipe out" a South Korean island as Pyongyang came under new economic and diplomatic fire from US sanctions and UN charges of gross rights abuses.

On a visit to frontline military units on Monday, Kim briefed officers on their mission "to strike" Baengnyeong and turn the island into a "sea of fire".

"Once an order is issued, you should break the waists of the crazy enemies, totally cut their windpipes and thus clearly show them what a real war is like," Kim was quoted as saying by the Korean Central News Agency.

Military tensions between Seoul and Pyongyang have risen to their highest level for years, with the communist state threatening nuclear war in response to UN sanctions imposed after its third atomic test last month.

An administrative official on Baengnyeong, Kim Young-gu, said civilian emergency shelters on the island had been fully stocked and all village councils put on high alert.

"It's not like there's a mass exodus of panicked islanders to the mainland. But to be honest with you, we're a bit scared," he told the AFP news agency by telephone.

The disputed sea border off the west coast was the scene of deadly naval clashes in 1999, 2002 and 2009.

North Korea has also announced its unilateral abolition of the 60-year-old Korean War armistice and non-aggression pacts with Seoul in protest at a joint South Korean-US military exercise that began on Monday.

'Tremendous sense of danger'

The crisis represents an early test for South Korea's new President Park Geun-hye, who was sworn in only two weeks ago, while analysts worry about just how far the inexperienced Kim Jong-un is willing to go.

A domestic political row has hindered key appointments to Park's cabinet, and the nominee for the defence portfolio, Kim Byung-kwan, warned that delaying his confirmation posed enormous risks.

"I feel a tremendous sense of danger," he told a televised press conference. "There should never be a slightest vacuum in national defence at any moment. And now is a perilous time."

Defence Ministry spokesman Kim Min-seok said the North was trying to exert "psychological pressure" on South Korea, and was expected to launch full-scale military manoeuvres in the coming days.

"If the North provokes us, we will respond in ways that will cause them more harm," he said.

Charges of rights abuses

In a move likely to provoke a fresh round of furious rhetoric from Pyongyang, the United States on Monday slapped sanctions on North Korea's primary foreign exchange bank and four senior officials.

Pyongyang came under attack on another front at the UN Human Rights Council in Geneva, where the UN special rapporteur on human rights in North Korea presented a a series of charges of abuses and crimes against humanity.

Rights violations in North Korea "have reached a critical mass", Marzuki Darusman told the council, citing public food deprivation, torture and arbitrary detention.

He highlighted concerns about a network of political prison camps believed to hold at least 200,000 people, and called for an international commission of inquiry into North Korea's human rights record.