The United Nations Tuesday opened a new office in Seoul to monitor North Korea's human rights record, prompting a warning from Pyongyang of "resolute toughest" action against what it termed a grave" provocation.

The office was formally opened in a ceremony attended by UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein, despite the North's repeated threats of "merciless punishment" against South Korea if the mission was launched.

The North has already announced it will boycott next month's World University Games in the southern South Korea city of Gwangju in protest at the office.

"Less than 50 miles from here lies another world marked by the utmost deprivation," Hussein said in a statement to mark the opening, referring to the North.

"The Seoul office will monitor and document human rights issues in (North Korea), building on the landmark work of the commission of inquiry and special rapporteur. We firmly believe this will help the basis for future accountability," he said.

Many North Koreans have escaped to find a new life in the South, but millions remain "trapped in the grip of a totalitarian system which not only denies their freedom but increasingly their basic survival needs", he added.

Human Rights Watch's deputy Asia director Phil Robertson described the new UN office as a "critical step forward" in the campaign to end North Korea's "systematic and pervasive human rights abuses".

About 20 activists rallied in Seoul against the opening of the mission, saying it would be used to "bring down" the North Korean government and would aggravate strained inter-Korean relations.

The United Nations proposed opening the field office following a searing report published last year by a UN commission, which concluded North Korea was committing human rights violations "without parallel in the contemporary world".

Based on the testimony of hundreds of North Korean exiles, the commission detailed a vast network of prison camps holding up to 120,000 people and documented cases of torture, summary executions and rape.

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The report formed the basis of a resolution adopted by the UN General Assembly urging the Security Council to consider referring Pyongyang to the International Criminal Court.

Pyongyang has described the report as a work of fiction authored by the United States and its allies.

"The DPRK (North Korea) will decisively foil the reckless 'human rights' racket against the DPRK through resolute toughest counteractions," a foreign ministry spokesman said in a statement on the official news agency Tuesday.

The UN office is "a hideous politically-motivated provocation" challenging the North's dignity and social system, it said.

"This is an anachronistic behaviour and a grave provocation pushing the inter-Korean confrontation to an extreme phase contrary to the desire of all Koreans for improved inter-Korean relations," the statement added.

Cross-border tensions have remained high this year due to a series of North Korean ballistic missile tests, nuclear threats and annual US-South Korean military exercises.

Pyongyang sees the joint drills, most recently carried out in March and April, as a rehearsal for invasion. South Korea and the US say they are purely defensive.