DAMASCUS - Half of the members of the United Nations observer mission in Syria have left the country, as it starts its "final" 30-day mandate, the UN's chief peacekeeper Herve Ladsous told reporters on Wednesday.

"UNSMIS (is) in a reduced format," he said of the United Nations Supervision Mission in Syria, speaking at a press conference in Damascus.

"About half the military observers have been for the time being sent back to their countries, so the mission operates on a reduced basis, reduced in numbers, reduced in team size in the provinces and does what it can," he said.

"But of course taking into account the security situation, which of course in many places is extremely delicate."

Earlier, two members of the observer mission said that about half the team of 300 unarmed military observers had quit Syria.

"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 mission, which consists of 300 unarmed military observers accompanied by around 100 civilian support staff, was deployed in April to oversee a ceasefire that went largely unrespected.

In mid-June, it suspended patrols as fighting intensified.

On July 20, the UN Security Council voted to extend the mission's mandate for a "final" 30 days.

Ladsous pointed out that the resolution says any extension of the mandate would only be considered if there was "very specific and sustainable progress on the level of violence, which should subside substantially, and on the use of heavy weapons.

"So this is what we have to bear in mind," he said.

The new head of the mission, Lieutenant General Babacar Gaye, also addressed reporters, noting he was taking over "in a very difficult situation."

"During our last trip in Damascus... we witnessed some decrease in the violence, but unfortunately since that we had to suspend our activities," he said.

"We are back with the hope that reason will prevail, that there will be in this tunnel some light that we can seize and obtain less violence," he said.

"We have 30 days and around today 27 to go, so every opportunity will be seized to alleviate the suffering of the population. This is our main concern."

Western governments have expressed scepticism about the chances that violence will subside within the period of the mission's 30-day mandate.

US ambassador Susan Rice said the resolution would simply allow the observers "to withdraw safely" from Syria, although British Foreign Secretary William Hague said it gave President Bashar al-Assad's government "the final chance" to keep its commitment to end violence.

But Russia's ambassador Vitaly Churkin insisted that the phrase "final period of 30 days" in the text was not the death knell for UNSMIS.

"This is not a resolution about withdrawal, it is a resolution about continuation of the activity of the mission," he said.