Damascus - Half of the members of the United Nations observer mission in Syria have left the country, two mission members said on Wednesday.
"One hundred and fifty observers left Syria on Tuesday evening and Wednesday and they will not come back," one observer said, speaking on condition of anonymity.
"They left after a decision was taken to reduce the mission by half," a second observer said, without specifying who had taken the decision.
The UN Supervision Mission in Syria (UNSMIS), as the force is officially known, consists of 300 unarmed military observers accompanied by around 100 civilian support staff.
It was deployed in April to oversee a ceasefire that went largely unrespected and in mid-June stopped carrying out patrols as fighting intensified.
On 20 July, the UN Security Council voted to extend the mission's mandate for a "final" 30 days, with Western nations warning that the continued violence meant it was unlikely the observers would be able to remain in country.
US ambassador Susan Rice said the resolution would allow the observers "to withdraw safely" from Syria, while British Foreign Secretary William Hague said the resolution gave President Bashar Assad's government "the final chance" to keep its commitment to end violence.
But Russian ambassador Vitaly Churkin insisted that the phrase "final period of 30 days" in the text was not the death knell for UNSMIS and that its work should continue.
"This is not a resolution about withdrawal, it is a resolution about continuation of the activity of the mission," he said.
Meanwhile, Sapa reported that at least 33 people were killed on Wednesday across Syria, mainly in the northern city of Aleppo where Assad's forces are carrying out a major attack against rebels, citing opposition activists.
"Helicopter gunships have bombarded neighbourhoods in Aleppo, being controlled by the revolutionaries, causing several deaths and injuries, the opposition Syrian Observatory of Human Rights said.
Abu Omar al-Halabi, a rebel Free Syrian Army commander, told dpa that reinforcements rushed to back government troops in Aleppo after rebels had made territorial gains across the city.