SEOUL - SOUTH Korea's main spy agency said on Friday it is not involved in an alleged plot to kill North Korean leader Kim Jong-Il, after Pyongyang's claims of an assassination attempt fuelled cross-border tensions.

The communist state said on Thursday it recently arrested a man who tried to conduct a 'terrorist mission' against Mr Kim under orders from a South Korean intelligence agency.

'It has nothing to do with the National Intelligence Service,' a spokesman for the NIS told AFP.

Seoul's unification ministry declined comment on the plot claim, which comes at a time of frosty relations between North and South.

Early this month the North expelled hundreds of South Korean workers from a joint industrial estate and imposed strict border controls, in protest at what it called the Seoul government's confrontational policy.

The North's State Security Ministry said a man surnamed Ri had been detained while carrying out a 'terrorist mission given by a South Korean puppet intelligence-gathering organisation to do harm to the safety of the top leader of the DPRK (North Korea).'

The ministry's statement, carried late on Thursday by official media, said Ri crossed the border early this year, without specifying where, and came into contact with a South Korean agent surnamed Hwang.

It said the intelligence organisation later sent him back to the North after training him to gather information about Mr Kim's official visits.

'The organisation also sent him speech and acoustic sensing and pursuit devices for tracking the movement of the top leader and even violent poison in the end,' said the statement.

'This case goes to prove that the South Korean puppets have gone the lengths of resorting to thrice-cursed methods to dare harm the headquarters of the DPRK, hell-bent on inciting confrontation with the DPRK.'

The North cited the Lee Myung-Bak government's 'anti-North Korean moves' as its reason for speaking out, saying these have reached 'an extremely reckless and dangerous phase.'

The state of Kim's health is being closely watched overseas after US and South Korean officials said he suffered a stroke around mid-August.

Admiral Timothy Keating, commander of the US Pacific command, said in Washington Thursday the 66-year-old remains in control of his government.

Analysts said it was unusual for the North to allege a plot to harm Mr Kim.

'This reflects the current status of inter-Korean ties,' Koh Yu-Hwan of Dongguk University told Yonhap news agency.

Baek Seung-Joo of the Korea Institute for Defence Analyses told AFP: 'The North is likely to use these alleged incidents as a pretext to heighten tension.'

The statement did not name the NIS as involved in the alleged operation against It said the North also foiled an attempt to collect samples indicating possible nuclear activity 'as instructed by a puppet intelligence-gathering and plot-breeding institution.'

International nuclear disarmament talks are stalled over the North's refusal to permit the taking of samples to check its atomic activities.

The security ministry said authorities also foiled a plot to gather secrets and lure top officials to defect which had involved 'a corrupt Korean woman residing in China as a spy.'

It said plans to form an underground church had also been foiled, as had several attempts to incite defections in a bid to set up a centre 'fabricating misinformation' about human rights abuses in the North.

Seoul-based private groups, including Christian organisations, work to help refugees flee North Korea and to collect data on rights abuses.

Inter-Korean relations have soured since President Lee's conservative government took office in February following 10 years of liberal rule.

Mr Lee has linked major economic aid to progress in the North's de-nuclearisation, a stance that enrages Pyongyang. -- AFP