At least 18 Sri Lankan army soldiers have died in an explosion that struck their vehicle in the rebel-dominated north-east of the country.
Military officials said a roadside bomb or a concealed mine blew up as the soldiers travelled through the district of Trincomalee.
Hours earlier, Sri Lanka's Tamil Tiger rebels declared a ceasefire "void" after clashes with government forces.
At least nine soldiers and 35 rebels are thought to have died in fighting.
Spokesmen for the Tigers were reported to have accused the government of restarting the country's civil war, which was halted by a 2002 ceasefire.
"Militarily, we have decided to fight back"
"It is the government which has started the war. Militarily, we have decided to fight back if the Sri Lankan army enters our area."
The government said its operations, aimed at securing water supplies, were "humanitarian", and denied breaking the ceasefire.
Simmering war
Analysts have warned the violence risked returning the country to all-out war.
"The country is entering a deeper state of conflict with ground troops on both sides fighting over territory in a sustained manner," analyst Jehan Perera told the Associated Press.
But the head of the monitoring mission in Sri Lanka, Ulf Henricsson, said a return to full-scale war would benefit neither side, and he expected a continuation of low-intensity conflict in the coming months.
More than 800 people are thought to have died this year in simmering violence throughout the country.
The rebels are seeking an independent Tamil homeland in the north and east of Sri Lanka.
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