SEOUL - SOUTH Korea is on guard against any military provocation by North Korea after the communist state ordered a border clampdown amid worsening ties, the defence ministry said on Wednesday.

The North on Monday imposed strict border controls and ordered the expulsion of hundreds of South Koreans working at a joint industrial estate, in protest at what it calls the Seoul government's confrontational policy.

'In response to the North's December 1 measure, surveillance and control operations are being stepped up against (any) naval attacks and attempts to kidnap fishing boats,' the ministry said.

Special training programmes are being implemented to cope with 'contingencies' along the heavily fortified land border, it said in a report to parliament.

Some analysts believe the North may provoke a limited clash around the disputed border in the Yellow Sea, the scene of bloody naval battles in 1999 and 2002.

The ministry report said the North in 2007 deployed new ballistic missiles with a range of 2,500-4,000 kilometres. It was also building up ground forces including tanks, artillery and special warfare troops.

A defence ministry spokesman declined to give details about the missiles.

The report also accused the North of breaching military agreements between the two sides, who have remained technically at war since their 1950-53 conflict ended only in an armistice.

'North Korea has breached or failed to honour most agreements reached between the South and the North in military affairs,' it said.

Ties have worsened since a conservative government came to office in Seoul in February, replacing 10 years of liberal rule, and took a tougher stance on cross-border ties.

On Monday, the North halted a cross-border cargo rail service and a day tour and began strictly restricting other border crossings. It has permitted only 880 South Koreans to work at the Kaesong industrial estate, half the number which Seoul says is needed to keep it running.

The curbs cast a cloud over the future of Kaesong, where around 35,000 North Koreans earning 70 dollars a month (S$159.2) work for 88 South Korean light industrial firms.

North Korea is also angry at propaganda leaflets floated by rights activists across the border. -- AFP