Security forces shot dead five members of the Liberation Tigers of Tamil Eelam (LTTE) in a jungle area around the eastern lagoon town of Batticaloa on Thursday evening, officials said.

The Sri Lankan military is trying to oust the LTTE from the east -- seen by the rebels as part of what should be their future separate state -- and confine them to a single area in the north of the island.

Military officials also said three civilians were killed by unidentified gunmen in the northwestern coastal area of Mannar, where troops claimed they killed 20 Tiger rebels during a 24-hour period ending Thursday morning.

Troops on Thursday recovered bodies of six of the members of the Liberation Tigers of Tamil Eelam (LTTE) in the Madhu area, about 260km north of Colombo, the ministry said.

"Since yesterday, 20 LTTE terrorists were killed during conflict with troops," the ministry said in a statement. There was no immediate word from the Tigers.

The military claims came after the army said that Tiger rebels shelled military positions along their de facto border in northern Sri Lanka Thursday, prompting the closure of the frontier post into rebel-held territory.

The Omanthai check point, which is at the entrance to the northern Wanni region that is dominated by the LTTE, closed indefinitely following Thursday's shelling, the army said in a statement.

"The closure will remain until the ground situation returns to normality," the statement said.

Government forces and the rebels have been trading long-range fire across their defence lines in the northern region where security forces have been making incursions into rebel territory in recent weeks.