North Korea does not appear close to conducting a nuclear or missile test despite continued preparations, a South Korean minister said Wednesday after a purge in Pyongyang sparked fears of aggression.

"Preparations have been made continuously... but I don't think a nuclear test or long-range missile launch is imminent," Unification Minister Ryoo Kihl-Jae told a parliamentary committee.

The minister also said no abnormal North Korean troop movements had been detected.

But he called for a continued watch for possible provocations following the shock execution last week of Jang Song-Thaek, the uncle and one-time political mentor of leader Kim Jong-Un.

Jang's purge raised questions about factional infighting at the top of the Pyongyang hierarchy and prompted both Seoul and Washington to warn of possible provocative acts by the nuclear-armed North.

His execution was just one of many in recent months, said Marzuki Darusman, the United Nations special rapporteur on human rights in North Korea.

"I have also received information from different sources about multiple reports of public executions in different locations in DPRK (North Korea), in particular during the past four months, which allegedly involved charges such as selling illegal videos, viewing pornography and taking drugs," Darusman said in a statement from Geneva, urging Pyongyang immediately to halt all executions.

US Secretary of State John Kerry, in an interview with ABC television aired Sunday, also said there had been "a significant number of executions" in recent months.

In October South Korea's spy chief said he was "aware" of the alleged execution of members of the North's national orchestra, said to include a singer rumoured to be Kim Jong-Un's former girlfriend.

And in November a South Korean newspaper reported that the North publicly executed around 80 people earlier in the month, many for watching smuggled South Korean TV shows.

The North is censured internationally both for its rights record and its weapons programmes.

It carried out nuclear tests in 2006, 2009 and in February this year, triggering virtually universal criticism from major countries and tight sanctions.

But the reclusive regime has vowed to strengthen its nuclear weapons programme, describing it as a deterrent to a hostile United States.

US researchers have observed work at the ageing plutonium reactor at Yongbyon, which would allow the North to expand its programme.

Its rocket launch in December was condemned by the international community as a disguised ballistic missile test that violated UN sanctions.

North Korea insisted it was a purely scientific mission and vowed to push ahead with similar launches in the future.

The North has expressed interest in a resumption of six-party talks on its nuclear programme, but the United States says it must first demonstrate a commitment to denuclearisation.