Nineteen soldiers were killed and two others wounded on Monday night in the bloodiest bus ambush against security forces since a truce went into effect in February 2002, officials said.
The soldiers were traveling from Allai to Kantalai in the district of Trincomalee when Liberation Tigers of Tamil Eelam ambushed them, military officials said.
The victims were on their way to reinforce government troops battling Tamil Tiger rebels in the same area.
It was the biggest single loss for the military in a mine attack.
Twelve sailors were killed in the same area in a similar mine attack on April 11. Another 12 sailors were killed in an identical ambush on December 23 at Pesalai in the northwest.
The biggest ambush against civilians was on June 15th when 64 bus passengers were killed.
In an unrelated incident, a police armoury caught fire after an accidental explosion at Katukurunda, about 35 kilometres south of Colombo. There were no immediate reports of casualties in that.
Police said an investigation was underway.
