The aid workers were found shot dead in Muttur in the north-east

The deaths this month of the Action Against Hunger workers were "a gross violation of the ceasefire accord by the security forces", monitors said.

They were "convinced" no other armed group could have been behind the killings near Muttur in the north-east.

A government spokesman angrily rejected the allegation, calling it "pathetic and biased".

To have a [civil] war, you must have two sides. At the moment, the government is merely responding to [Tamil Tiger] aggression Mangala Samaraweera, foreign minister Full text of statement

Keheliya Rambukwella told AP news agency: "We deny it and it's a totally baseless statement that the head of the [monitoring mission] has made.

"They have no right to make such a statement because they are not professionals in autopsy or post-mortem."

The aid staff - all but one ethnic Tamils - were working on tsunami relief projects in an area that had seen several days of fighting between Tamil Tiger rebels and government troops.

Outcry

A statement released by Maj Gen Ulf Henricsson, the head of the Sri Lanka Monitoring Mission, said he had held "confidential conversations with highly reliable sources" on who was most likely to have been responsible for the killings.

"The views have not proved contradictory and the security forces of Sri Lanka are widely and consistently deemed to be responsible for the incident," he said.

Monitors also blamed Tamil Tigers for a deadly bus attack in June

Fifteen of the bodies were found lying down and shot at close range on 7 August in a case that caused an international outcry.

Two other bodies were found later.

The monitors said that after interviews with officials and witnesses they were "convinced that there cannot be any other armed groups than the security forces who could actually have been behind the act".

The statement called the incident a "committed act of assassination" that was "one of the most serious recent crimes against humanitarian aid workers worldwide".

The statement also covered two other areas in the conflict and determined that

• Government troops or armed elements aided by them were responsible for claymore mine attacks in the Mannar and Vavuniya districts from 1 April to 15 June - a "deliberate strategy against Tamil Tiger cadres and civilians" and a "gross violation" of the ceasefire
• It was "highly probable" that Tamil Tigers or supporters carried out a bus attack that killed more than 60 people in Kabithigollewa, 200km (125 miles) north of Colombo in June.

A rebel spokesman told the BBC Tamil service that the monitors were wrong and said they had had nothing to do with the bus attack.

The ceasefire was sealed in February 2002 but in the past few months the two sides have engaged in open conflict in the north and east.

However, on Wednesday Sri Lankan Foreign Minister Mangala Samaraweera denied his country was in a state of civil war.

"To have a [civil] war, you must have two sides. At the moment, the government is merely responding to [Tamil Tiger] aggression," he told the BBC.

European Union nationals in the monitoring mission are now completing a pullout which was demanded by the Tamil Tiger rebels after the EU listed them as a terrorist organisation.

Of the 57 original monitors, only about 20 will remain.

Meanwhile, the number of refugees who have fled Sri Lanka's conflict for the southern Indian state of Tamil Nadu has crossed the 10,000 mark, officials there confirmed.