Sri Lanka says its troops have killed more than 100 Tamil Tiger rebels as new clashes flared on three fronts.
Fighting broke out as the sun began to rise on Saturday
Fierce battles broke out in the Jaffna peninsula in the north and in the east at Batticaloa, while the north-eastern port of Trincomalee saw heavy shelling.
Sri Lanka has been wracked by fighting between rebels and government forces in recent months, with hundreds killed and an estimated 50,000 forced to flee.
Meanwhile, a senior government peace official has been shot dead in Colombo.
Police in the capital said Ketheesh Loganathan was gunned down at his home in the capital's Dehiwala suburb.
He was the deputy head of the government's Peace Secretariat, which co-ordinates the Norwegian-brokered peace process between the government and rebels.
Sri Lanka's key foreign donors have called for an immediate end to the hostilities, which they said was "seriously unravelling" the 2002 ceasefire agreement.
A joint statement by the US, European Union, Japan and Norway also said they were deeply concerned about the growing humanitarian crisis and urged both sides to allow relief agencies access to disputed areas.
Saturday saw heavy artillery exchanges and both sides said they had taken or recaptured territory.
Some of the worst fighting was reported in Jaffna, where thousands of government soldiers are stationed on the isolated peninsula, cut off by rebel territory and accessible only by sea.
Spokesmen for the Liberation Tigers of Tamil Eelam (LTTE) insisted that government troops began attacking rebel forces in Jaffna, but said their units were fighting back strongly.
But the government accused the rebels of sparking the fighting.
"The LTTE is definitely pushing the country to war," defence spokesman Keheliya Rambukwella told a news conference.
The military said 27 troops were killed and 80 injured, and estimated that between 100 and 150 Tamil Tigers had died.
The military reported rebel shelling of positions in the north-eastern port of Trincomalee.
Sri Lanka's air force also bombed rebel positions near the eastern coastal town of Batticaloa.
One official described the air raid, the latest in a series of air strikes against Tamil Tiger positions in recent weeks, as a "limited air attack".
Fighting around Trincomalee in recent weeks has hinged on a contested water supply.
Government forces have vowed to regain control of the water supply from the rebels, who they accuse of deliberately restricting supplies to Sinhalese farmers in government-held territory.
On Friday, a Tamil Tiger spokesman told the BBC that the government had opened up a "new front" in the simmering conflict by launching waves of air attacks in the east of the country.
Sri Lanka's air force confirmed it bombed rebels (Photo: Sri Lankan air force)
Seevarathnam Puleethevan said many of the group's fighters had been killed by "heavy aerial bombardment".
The fighting began last month and has seen some of the worst violence in Sri Lanka since a 2002 ceasefire came into force.
Both sides maintain they are still abiding by the terms of the ceasefire and are responding defensively.