At least five Sri Lankan soldiers and two civilians have been killed in a mine attack in the northern Jaffna peninsula, the army says.
The troops were on patrol when the claymore mine exploded, army spokesman Brigadier Prasad Samarasinghe said.
Tamil Tiger rebels are suspected to be behind the attack, he said. The rebels have denied being involved.
The government and the rebels are due to meet in Geneva later this month for a second round of peace talks.
The violence comes as ambassadors from the European Union (EU), Japan and Norway met rebel political leaders in their headquarters in the northern town of Kilinochchi.
Brig Samarasinghe said two soldiers were injured in the blast, which took place at Mirusuvil in Jaffna peninsula.
Army says the rebel attacks are violations of the truce
The civilian victims were Tamil humanitarian workers with an international Catholic aid agency - Caritas' Human Development Centre - who were travelling in another vehicle coming from the opposite direction, he said.
"These are clear violations of the ceasefire as well as agreements reached in Geneva. the Tigers promised in Geneva to stop all attacks against the troops", Brig Samarasinghe said.
Denying involvement, the rebels have condemned the attack.
"We really sympathise with the people who were injured and who died," rebel spokesperson Daya Master told Reuters news agency.
Hours earlier, a grenade exploded near the newly-opened office of a breakaway rebel faction in the eastern town of Batticaloa, Brig Samarasinghe said.
No-one is reported to have been injured in the blast.
The visiting EU ambassador Julian Wilson told Reuters that the death of the aid workers was "particularly horrific".
"The fact there is an attack and deaths on the same day we are actually speaking rather colours the issue," he said.
There has been a string of attacks since both sides held a round of talks in Geneva in February.
About two weeks ago, the army reported that six rebels had blown themselves up off the north-western coast near a navy vessel. Eight sailors have been missing since the incident. The rebels denied any involvement in the incident.
A Tamil activist was also killed last week in the eastern town of Trincomalee. It is not clear who carried out the killing.
Previous such attacks have been blamed by the rebels on armed paramilitary forces allegedly backed by government forces, a charge which the military denies.
More than 120 people - including about 80 soldiers and sailors, and many civilians - were killed in the months of December and January, raising fears of the ceasefire breaking down.
Attacks are routinely blamed on the Tamil Tiger rebels but they usually deny any involvement in it, describing them as a "popular uprising" of the Tamil people.
But BBC correspondents say that few believe that such attacks can take place in rebel-held territory without their knowledge.
Tamil Tiger supporters say more than 40 Tamils have been killed by the security forces in a series of attacks since the start of December. Others blame some of those deaths on the rebels or other armed groups.
The government and the rebels are scheduled to hold the second round of talks in Geneva between 19 and 21 April to salvage the ceasefire.
More than 60,000 people have died during two decades of conflict in Sri Lanka.
The Tamil Tigers want autonomy for minority Tamils in the north and east.