The Tamil rebels said that Sri Lanka’s military killed thirteen civilians in a bomb attack in Killinochchi on Tuesday, just a few hours before the rebel leader’s annual speech.

The attack was triggered by a roadside bomb that was activated near a bus that was traveling inside the rebel stronghold and killed 11 schoolchildren and two adults.

It is known that such mines are used by the rebels against government forces.

According to military spokesman, Brig. Udaya Nanayakkara, the military didn’t aimed at the students.

The military is on alert for any attack from the rebels during the Tigers' "heroes' week" celebrations. They will end this Tuesday with the annual statement made by rebel leader Velupillai Prabhakaran.

The schoolchildren were killed as they traveled to a cemetery to take part in the annual Hero's Day memorials held for dead rebel fighters, Reuters reports.

Selvy Navaruban, Tiger spokeswomen on humanitarian issues and human rights said: "Thirteen civilians including 11 school children were killed by a Sri Lanka army deep penetration unit claymore attack in Iyankilam in (the) west of Kilinochchi.”

The military said that it didn’t have units in that area and that it had no responsibility for the attack.

Nanayakkara said: “How can we go to Kilinochchi. It's not possible. They must have done it.” He added that it could have been a rebel mine or activated by other group.

The blast occurred at 15 miles west of the Tigers' de facto capital of Kilinochchi.

The other two adults were the driver and one adult.

No independent confirmation was made.

The Tamil Tiger rebels say that they fight for an independent state in the north and east of the island.

Since early 2006 almost 5,000 people got killed in the clashes between the military and the LTTE and 70,000 people died since the war erupted in 1983.