The Britain-based Syrian Observatory for Human Rights called on the observers to "either work on stopping the killings or leave". The group said the observers' role has become to "witness the killing in Syria" rather than implement Annan's six-point plan.
It was the harshest criticism of the observers by an activist group since they started their work in April.
Some analysts say that even if the number of observers increased by tenfold, it still would be very difficult for them to report and investigate acts of violence throughout the country.
"Even 3000 observers will not be enough," said Hisham Jaber, a former Lebanese army general who heads the Beirut-based Middle East Centre for Studies and Public Relations.
Still, the presence of the observers is considered critical to understanding the conflict in a country where the government prevents reporters from operating independently.
Mood said all the observers are frustrated because the violence has not only persisted but actually increased.
On Friday, the Syrian regime kept up a ferocious offensive on rebel areas around the country in one of the most serious escalations in violence since Annan brokered the nominal truce.
An activist in the northern city of Aleppo said troops backed by helicopters and tanks were engaged in "raging battles" in the rebel-held town of Anadan, and in other towns in the district.
Elsewhere, activists said Syrian troops shelled the central city of Homs, which has come under attack for days.
The violence did not stop thousands of Syrians from taking part in demonstrations against President Bashar Assad yesterday.
-AP
