European ceasefire monitors on Tuesday accused Tamil Tiger rebels of violating Sri Lanka's fragile truce by detaining three policemen and refusing them access to the men.

"This is a very serious matter," said Helen Olafsdottir, a spokeswoman for the team monitoring the ceasefire agreement between government forces and the guerrillas.

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Three male and two female police officers, two Catholic priests and a civilian were detained by the Tigers on Friday, after they ventured into rebel-controlled territory in Mannar. All but the three police officers have been released.

Mannar, about 220 kilometres north of Colombo, is a government-held town but the rebels have pockets of control.

The group of eight, attached to Sri Lanka's child protection authority, said they were tracking a pedophile allegedly hiding in rebel territory when they were detained by the rebels.

Truce monitors slam LTTE over violations

"The policemen should have never been detained. These people were unarmed and chasing an internationally wanted criminal," Olafsdottir said, adding, "It's a violation of the cease-fire and adds to the deadlock the two parties are already in."

She said the monitors had demanded the immediate release of the detainees.

The rebels have denied access to the men, pending their trial before a rebel court on September 27. They will be tried for entering rebel-held territory without permission. The Tigers run their own administration in areas under their control in the Tamil-majority north and east.