The statement said: "SLMM [the monitoring mission] is, with the obtained findings, convinced that there cannot be any other armed groups than the security forces who could actually have been behind the act."
In response to the report, Brigadere Prasad Samarasinghe, a Sri Lankan military spokesman, told Aljazeera that an investigation was being carried out by the ministry of defence and the inspector general.
"I can not comment on an ongoing investigation," Samarasinghe said.
The victims, all but one of them ethnic Tamils, were found with gunshot wounds, lying face down in the compound of their office clearly identified by their Action Contre La Faim (ACF) T-shirts.
The SLMM concluded that there were no other forces in Muttur at that time, and blamed the army for trying to cover up the killing by restricting movement of the SLMM in the region.
"The views have not proved contradictory and the security forces of Sri Lanka are widely and consistently deemed to be responsible for the incident," the report said.
"Eight monitors have left during this week, and another 22 are scheduled to leave in the coming days, which leaves the mission with only 20 Icelandic and Norwegian monitors"
SLMM weekly report, August 27
The SLMM also ruled that a fragmentation mine attack on a civilian bus in June that killed almost 70 people was a breach of the ceasefire by the Liberation Tigers of Tamil Eelam (LTTE), while blaming the government for a string of similar attacks in rebel areas since April.
Ahead of the September 1 deadline issued by the Tigers for all European monitors to leave the island, after the EU declared the LTTE a terrorist organisation, the SLMM said: "The mission was continuing to prepare for the withdrawal of Danish, Finnish and Swedish monitors.
"Eight monitors have left during this week, and another 22 are scheduled to leave in the coming days, which leaves the mission with only 20 Icelandic and Norwegian monitors."
Norway is not a member of the EU.
The report also said that the governments of Iceland and Norway have decided to increase the number of monitors to 30 as soon as possible.
The army launched its heaviest artillery barrage for days towards rebel territory south of Jaffna early on Wednesday, but some schools in the town opened for the first time since the siege began two-and-a-half weeks ago.
"I can hear the shells and I'm a little worried," said civil servant Suresh Kumar as he delivered his son to school. "If things are bad tomorrow I won't let him come. Their life is more important than these exams."
A Tamil journalist who was abducted ealier this week has been released unharmed.
The military was continuing its offensive on the Jaffna peninsula to secure Trincomalee.
A military spokesman said: "The troops are moving towards Sampur [on the southern lip of Trincomalle harbour]. The main thing of this operation is to capture it to secure Trincomalee harbour and nearby civilian areas."
The military and the LTTE are
accused of breaking the ceasefire
Officials said Mahinda Rajapakse, the president of Sri Lanka, and Mangala Samaraweera, the foreign minister, had flown to London to meet the British prime minister, but had no details.
At least 65,000 people have died in the war and up to 200,000 people have been internally displaced.