Colombo - Security forces have mounted a search operation in north-central Sri Lanka after Tamil separatist rebels gunned down four farmers overnight and wounded four others, a military official said Thursday.

The farmers were shot in the Menikdeniya area of Welikanda, 160 kilometres north-east of Colombo late Wednesday.

The incident was the second such attack in less than 24 hours. On Tuesday night, five farmers were shot dead and two more injured in Serunuwara, 230 kilometres north-east of the capital.

The attacks came as government troops continued their offensive to recapture what they call the last rebel-controlled areas in Sri Lanka, also in the north-east.

Defence spokesman Keheliya Rambukwella said at a news conference that the rebels of the Liberation Tigers of Tamil Eelam - who have been fighting for more than 25 years for a separate state for the Tamil ethnic minority in the northern and eastern parts of majority-Sinhalese Sri Lanka - were now confined to an area of 21 square kilometres in the Mullaitivu district.

He claimed that of the 21 square kilometres, 20 were in a "safe zone," which was unilaterally declared by the government for civilians to take shelter.

Tamil rebels have claimed that the "safe zone" has been under fire from the security forces with regular incidents where civilians have been killed or injured.

The area has also been short of regular medical and food supplies although limited supplies are being sent through by ship under the International Committee of Red Cross flag. The food is being unloaded in a coastal area where the safe zone is located. (dpa)