Brigadier Udaya Nanayakkara, Sri Lankan military spokesman said clashed with fighters in Tamil Tiger-held territory in the Mullaitivu, Vavuniya, Mannar and Welioya regions.
However, the LTTE, which is fighting for an independent state for ethnic minority Tamils in the north and east of the country, said it had thwarted the offensive in Mallavi and Mullaiteevu.
"At least 30 SLA soldiers were killed and more than 60 troopers sustained injuries in an intense fighting. Three dead bodies of SLA soldiers were also recovered by the Tigers," a statement from the group on a pro-LTTE website said.
The website also posted pictures of dead bodies of the three soldiers and weapons captured during the operation.
Sri Lanka's military also reported that Tamil Tiger fighters attacked troops in the eastern district of Ampara, where the military and the government had said they had wiped out the LTTE last year.
Independent verification of the fighting is not possible because journalists are barred from the war zone and casualty figures reported by the sides differ wildly.
The Tamil Tigers said it had declared a 10-day unilateral truce starting from July 26 as a goodwill gesture for the summit in Colombo. But the government dismissed the announcement saying it had not received official notification and was sceptical about the declaration.
The two sides have been fighting for more than two decades and the conflict has claimed the lives of more than 70,000 people in that time.
Leaders from Afghanistan, Bangladesh, Bhutan, India, the Maldives, Nepal, Pakistan and Sri Lanka are expected to approve a series of measures aimed at combatting terrorism, including freezing funds.
The summit is also set to approve the creation of a regional development fund and a food bank to cope with shortages caused by soaring prices.