Unidentified snipers in Syria have shot at and damaged a vehicle being used by UN chemical weapons investigators trying to reach the site of a suspected poison gas attack.

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The UN convoy of six vehicles came under fire on Monday in the buffer zone between rebel and government territory near Damascus as it travelled towards Ghouta, the site of the suspected attack last Wednesday. 

"The first vehicle of the investigation team was deliberately shot at multiple times by unidentified snipers in the buffer-zone area," UN spokesman Martin Nesirky said. No one was injured and a replacement vehicle was obtained, he added.

Western powers claim that potential evidence in the area has probably been destroyed by heavy government shelling over the past five days.

The six-car convoy of chemical-weapons experts wearing blue UN body armour was accompanied by a car of security forces as well as an ambulance. 

News agencies later reported that the convoy had continued and had reached Moadamiyeh, one of the areas hit by the alleged chemical attack. Al Jazeera could not confirm those reports.

The alleged chemical-weapons attack in Ghouta killed 355 people and injured thousands, according to the aid group Medecins Sans Frontieres.

The UN announced on Sunday that the Syrian government and rebels had agreed to observe a ceasefire in the Ghouta area during the visit.

It said the rebels and government were responsible for the safety of the inspectors on the ground.

The agreement between the Syrian foreign minister and the head of the UN delegation "is effective immediately and it will allow UN delegation to investigate allegations of using chemical weapons on August 22 in Damascus suburbs", Syrian state TV reported.

On the diplomatic front, Francois Hollande, the French president, told his US counterpart Barack Obama on Sunday that he concluded that the Syrian regime was behind the attack.

"The two presidents agreed to stay in close contact to arrive at a joint response to this unprecedented aggression," Hollande's office said.

Remarks of William Hague, the British foreign secretary, on the alleged use of chemical weapons in Syria

William Hague, the UK foreign secretary, has said that any evidence of a chemical attack may have been destroyed.

"The fact is that much of the evidence could have been destroyed by that artillery bombardment," he said on Sunday.

The government of Bashar al-Assad has denied responsibility for Wednesday's attack and blamed the rebels fighting Assad's forces of the same.

The rebels have held the regime forces responsible for the attack.

The alleged chemical weapons attack has spurred calls for global action against Assad's government.

The US government has been under mounting pressure to act, with Obama having defined the use chemical weapons as a "red line' for Syria.

Source: Al Jazeera and agencies