SEOUL (AFP) - North Korea does not appear close to conducting a nuclear or missile test despite its continued preparations, a South Korean minister said on 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.

North Korea has 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.

The North's 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.