Sri Lanka's army described its actions in the island's east as "defensive" and said they were intented to "neutralise artillery and mortar fire", military sources said on Friday.
Thorfinnur Omarsson, spokesman of the Sri Lanka Monitoring Mission, said: "It is totally unacceptable if the military penetrate Tiger territory. That is an offensive right after agreeing to talks."
The military said the Tigers had attacked its frontline positions in the eastern district of Batticaloa and in the besieged army-held northern Jaffna peninsula, which is cut off from the rest of the island by Tiger-held territory.
The armed separatist group has threatened to withdraw from the truce completely if attacks by the military continue, but the government says it reserves the right to retaliate if the Tigers attack security forces.
"The Tigers fired artillery and mortars at our forward defence lines, and the military wants to neutralise them and push them back [to the east]," a military source said on condition of anonymity.
The operation comes only weeks after a military offensive to capture rebel territory on the southern lip of the strategic northeastern harbour of Trincomalee.
Sri Lankan forces also described this action as "defensive".
SP Thamilselvan, leader of the Tigers' political wing, said separatists expected little to come out of talks planned for the end of the month, and said violence would spread across the island if attacks by the military continued.
"Although we do not hold high hopes, we are ready to respect the call [for talks] and give it a chance," Thamilselvan said on Friday.
He said: "As far as we are concerned, when our homeland is crushed, and our people's habitats are destroyed, this war will definitely spread throughout the island."
The government says the talks will take place in Geneva on October 28 and 30.
The Tigers wanted the venue to be Oslo, and have not yet agreed on the current chosen location.
Each side accuses the other of trying to rekindle a conflict that has killed more than 65,000 people since 1983, and Nordic truce monitors see little will from either side to halt the violence.