COLOMBO, Sri Lanka — Tamil Tiger rebels said yesterday that they have killed at least 26 Sri Lankan sailors and caught four others alive in a fierce sea battle off the northern Jaffna peninsula.The fighting came as the United Nations and a rights group denounced the Sri Lankan military’s shelling of a school in which at least 23 Tamil civilians were killed and scores injured.

“The UN condemns in the strongest possible terms the shelling by the security forces of the government of Sri Lanka on defenceless civilians sheltering in Kathiraveli School in Vaharai yesterday,” the UN said in a statement.

“The killing and wounding of displaced persons is unjustifiable and a violation of the most basic humanitarian norms,” the statement said.

The UN added that it is disturbed by reports that the rebels have prevented some 2,000 civilians from fleeing to safety.

The Tigers have “an obligation to protect civilians and allow them freedom of movement to leave an area where they fear for their safety,” it said.

Of yesterday’s battle, the rebels said they caught four sailors alive and found the dead body of another. “Our fighters have confirmed that they killed at least another 25 sailors,” Tamil Tigers’ military spokesman Rasiah Ilanthirayan told The Associated Press.

He said five Sea Tiger rebel fighters also died in the attack, in which they sunk one of the navy’s fast attack craft and severely damaged another.

Military spokesman Brig. Prasad Samarasinghe said yesterday’s clash erupted when the rebels tried to attack a ship transporting some 300 civilians from the eastern port town of Trincomalee to Jaffna.

He said the navy, assisted by the air force, destroyed 22 of 26 rebel boats which had come to attack the ship, but he did not give any casualty figures.

Rebel spokesman Ilanthirayan denied that the rebels tried to attack a civilian ship.

“These are baseless allegations without any logic,” he said.

He said navy vessels disturbed the rebels’ “routine naval exercises” and the clash was “a result of that provocation.”

There was no independent account of the incident.

Separately yesterday, the military said Tamil rebels detonated a roadside bomb, killing two soldiers in northern Jaffna peninsula.

Meanwhile, the government said it regretted Wednesday’s artillery attack on the school.

“While we regret this whole episode, we say that national security is utmost,” chief government spokesman Keheliya Rambukwella told a news conference.

Scores of civilians had taken refuge in the school in Kithiraveli — a rebel-controlled eastern village — from fighting between the army and Tamil guerrillas when it was hit, said senior rebel official Seevaratnam Puleedevan.

He said at least 60 civilians were killed and 150 wounded.

Helen Olafsdottir, a spokeswoman for the European cease-fire monitoring mission, said monitors had counted 23 bodies, mostly of women, children and older people. Another 137 people were admitted to hospitals, she said. But she added the monitors did not find any sign of a rebel military installation in the impact area.

But government spokesman Rambukwella said it was likely that the rebels had forced civilians to stay in the area to become human shields — a charge the insurgents denied.

London-based Amnesty International also condemned the attack and called for an inquiry by international and independent human rights experts, saying there was an urgent need to “respond to the dramatic deterioration of the human rights and humanitarian situation.”

In Colombo, 22 deputies from the Tamil National Alliance — a political party widely believed to be a proxy of the rebels — demonstrated in front of the UN refugee agency’s office, asking it to take action to protect displaced people.

The group said 41,000 Tamil civilians have been displaced in the Batticaloa region, the area of Wednesday’s attack.

But Samarasinghe said the military wasn’t targeting civilians. He acknowledged, however, that it had fired artillery to silence rebel guns.

Peace broker Norway, meanwhile, called on the government to explain why troops fired grenades that landed just metres from chief European cease-fire monitor Lars Soelvberg on Wednesday as he was visiting the village of Pooneryn in the country’s volatile north.

Norwegian Aid Minister Erik Solheim called the incident “very worrying” and asked the government for an explanation. He escaped unhurt. The military denied knowledge of Soelvberg’s presence in the area when they attacked.

The Tigers have been fighting since 1983 for a separate Tamil homeland in the country’s north and east, citing discrimination by the majority Sinhalese.

The conflict killed more than 65,000 people before a Norwegian-brokered truce in 2002, but subsequent peace talks failed. Surging violence this year has killed more than 2,000 civilians, soldiers and rebel fighters and threatens a return to all-out civil war.