Two Sri Lankan soldiers and a civilian have been killed in a roadside bomb attack, military officials say.
The end of the ceasefire is likely to bring an escalation in violence
Four other people were wounded in the attack on a van in the north of the island, close to the front-line with Tamil Tiger rebels, officials said.
The attack happened as Japanese peace envoy Yasushi Akashi was due to meet members of the government.
Fighting in Sri Lanka has worsened since the government said it was calling off a ceasefire with rebels.
The government is to formally pull out of the 2002 truce deal on 16 January.
Military spokesman Brigadier Udaya Nanayakkara told reporters that the latest attack had targeted a van carrying soldiers and civilians.
"This is definitely by the Tamil Tiger rebels," he said.
The rebels have not commented on the incident.
'All out war'
Mr Akashi's is the first visit by foreign mediators since the government announced it was withdrawing from the deal.
Both sides have repeatedly ignored the ceasefire. Some 5,000 people have been killed in fighting in the past two years alone.
The government has dismissed a call by the rebels for both sides to observe the truce, saying the Tigers used it as a cover to "unleash terror activities".
Observers are now forecasting a further escalation in the conflict.
Sri Lanka's army commander Lt Gen Sarath Fonseka has said his aim is to eliminate the Tamil Tigers before he leaves office at the end of the year.
The rebels say they are ready for all-out war.