Tamil Tiger rebels in Sri Lanka say more than 20 civilians, most of them children, have been killed in two attacks by the military in the north.
At least 11 of those killed were schoolchildren whose bus hit a mine laid by the military, the rebels said. The military denied responsibility.
Nine others died when the Tigers' radio station was bombed, the rebels said.
Earlier, the Tamil Tiger leader, Velupillai Prabhakaran, said hopes for peace were naive.
Fighting between troops and the rebels, who want autonomy for minority Tamils in the north and east, has worsened in recent months.
Observers say the two sides are gearing up for major confrontation in the north.
According to the rebels, the children killed by the mine on Tuesday were on their way to a ceremony to remember the rebels' war dead.
"Thirteen civilians, including 11 schoolchildren, were killed in a claymore attack, by the Deep Penetration Unit of the Government of Sri Lanka, on a van near Iyankulam, 25km west of Kilinochchi town," the rebels said.
The driver and an adult accompanying the children were also killed, a statement said. Pictures on the rebels' website showed the bodies of schoolgirls laid out on the ground.
The rebels are frequently accused of using claymore mines against government forces.
Sri Lanka's military denied carrying out the attack, saying they had no ground units in the area where the attack took place.
The rebels mark what they call Heroes' Day every year on 27 November.
Just an hour before the address from Tamil Tiger leader Prabhakaran was due to begin, Sri Lanka's air force flattened a clandestine radio station near Kilinochchi in the rebel-held north.
Nine people, five of them radio station employees, were killed, the rebels said.
The speech was broadcast, nevertheless, with Prabhakaran saying it was naivety to believe peace was possible with any of the parties in the Sinhalese-dominated south of the country.
He described the government as "genocidal" and said the international community should stop propping it up with economic and military aid.
Since his last address the Tigers have been driven from the east of the country and are under pressure in areas of the north that they still control.
But they have also unveiled new tactics deploying light planes, modified to carry bombs on night-time raids, the BBC's Roland Buerk in Colombo reports.
On Monday, the government marked Prabhakaran's 53rd birthday with a vow to kill him.
Defence secretary Gotabhaya Rajapaksa said the killing of the Tigers' political leader in an air strike earlier this month sent "a very powerful message".
"They know we have good intelligence on their movements," he told the AFP news agency.
"We are after him [Prabhakaran]. We are specifically targeting their leadership."
A Norwegian-brokered ceasefire in 2002 broke down two years ago, resulting in renewed fighting that has killed more than 5,000 people.
At least 70,000 people have died since the war began in 1983.