COLOMBO, July 7 (Reuters) - Sri Lanka beefed up security on Monday and warned of possible rebel attacks around the capital Colombo as the island's Tamil Tiger rebels marked the 21st anniversary of their first rebel suicide attack.

The Liberation Tigers of Tamil Eelam (LTTE) on the weekend commemorated the 356 men and women who have blown themselves up suicide attacks since the first such attack on 5th July 1987, when the suicide bomber or Black Tiger drove an explosive-laden truck into a Sri Lankan Army garrison in northern Jaffna.

"According to the information bureau, the LTTE is planning disruptions in south - Colombo, the suburbs and other parts. We have put all police officers on alert," said police spokesman Ranjith Gunasekera.

Last year, Black Tiger commandos attacked the Sri Lankan airbase in north central district of Anudradhapura in the rebels' first combined Black Tiger and air attack, destroying several aircraft.

Separately, Sri Lanka's military said it killed 69 Tamil Tiger rebels in the island's far north in fresh weekend fighting. Five soldiers were also killed.

The fighting in the northern districts of Jaffna, Vavuniya, Mannar and Polonnaruwa came amid near daily land, sea and air attacks, as the government tries to gradually retake the rebels' northern stronghold in a bid to win the 25-year civil war.

"Troops killed 69 LTTE terrorists and injured 77 in fighting since Friday. Five soldiers died and 23 were injured from the fighting," said a military spokesman.

The military also said troops captured strategically important rebel bunkers in island's north on weekend while air force gunship helicopters attacked rebel positions.

The LTTE, fighting to create an independent state in north and east Sri Lanka for ethnic Tamils, a minority in the predominantly Sinhalese country, were not immediately available for comment.

Analysts say the military has the upper hand in the latest phase of the long-running war given superior air power, strength of numbers and swathes of terrain captured in the island's east, though they still see no clear winner on the horizon.

An estimated 70,000 people have died since the civil war began in 1983. (Reporting by Ranga Sirilal; Editing by Valerie Lee)

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