The defence ministry said the action was a limited operation and not a return to full-scale war.
Officials said the attacks were aimed at ending a Tiger blockade of an irrigation canal that had deprived water to thousands of farmers in the island's restive northeast.
"The security forces are currently engaged in a limited operation with a clearly defined objective of securing water supplies to the civilian population," a statement from the ministry said.
Meanwhile, a regional Tamil Tiger (LTTE) leader asked the Swedish-led Sri Lanka Monitoring Mission (SLMM) on Sunday to declare that Sri Lanka's troubled truce was officially over after the week's bombing campaign by Sri Lankan aircraft.
"It is now appropriate for the SLMM to declare publicly that the ceasefire agreement is not holding any more on the ground," LTTE's regional leader told the pro-rebel Tamilnet.com website.
The LTTE has demanded that observers from European Union members Finland, Denmark and Sweden leave the island after the EU added it to a list of "terrorist" organisations in May. That would leave only Norwegian and Icelandic monitors.
Two policemen were shot dead in Trincomalee on Friday by suspected Tiger gunmen, police said, pushing the death toll to at least 910 since violence flared in December 2005.
Norway's Jon Hansse-Bauer, special envoy to Colombo, will visit the island next month to try to salvage the ceasefire, diplomats said.
Britain's deputy high commissioner, Lesley Craig, met the Tiger leadership in the rebel-held town of Kilinochchi on Friday and asked them to honour the ceasefire and move towards negotiations, the high commission said.
"We stressed the need for dialogue," the mission said after Craig held talks with the leader of the Tigers' political wing, SP Thamilselvan.
More than 60,000 people have been killed in 30 years of the Tamil separatist conflict.