The Liberation Tigers of Tamil Eelam (LTTE) killed the six men, including a senior officer, at Thoppigala, an area of jungle where the rebels maintain a stronghold, military officials said.

"Five men and their commanding officer were killed and seven escaped with injuries," a military official in the area said by telephone. "We don't have reports of Tiger casualties yet."

The killings came as the air force carried out two bombing sorties in the same region to support ground troops.

The defence ministry said the air attacks were to back infantry trying to wrest control of Thoppigala. A third bombing sortie was in the northwest Mannar region.

There were no immediate reports of casualties and there was no comment from the rebels, who on Thursday marked their "Black Tiger" day -- the anniversary of their first suicide bombing staged 20 years ago.

The LTTE released pictures of its chief, Velupillai Prabhakaran, dressed in jungle camouflage military fatigues and offering a flower garland to the portrait of the first Black Tiger, a guerrilla known as Captain Miller.

Miller drove a truck bomb into an army camp on the Jaffna peninsula on July 5, 1987, killing 40 government soldiers.

The guerrillas said 322 members of the elite Black Tiger suicide units had perished in attacks so far.

The latest Black Tiger anniversary came amid an upsurge of fighting on the tropical island. More than 5,000 people have been killed in the new wave of bloodshed that began in December 2005.

The Tamil Tigers are fighting for an independent homeland for the minority ethnic Tamils on the majority Sinhalese island. The 35-year-old conflict has claimed more than 60,000 lives.