The UN team in Syria will begin its mission on Monday to investigate claims of chemical weapons use, the UN spokesperson's office said.
The UN's mission has been given a green light by the Syrian government following talks with UN disarmament officials over the weekend.
The team had been on the ground for three days to probe earlier allegations when an apparent chemical attack in the Ghouta region near the capital Damascus killed at least 1,300 people.
A UN statement on Sunday said its secretary general had instructed the mission under Dr. Sellström, currently in Damascus, to "focus its attention on ascertaining the facts of the 21 August incident as its highest priority".
"All those in Syria have a stake in finding out the truth. The whole world should be concerned about any threat or use of chemical weapons," Secretary General Ban-ki Moon said.
Western countries, especially the US and the UK, seem convinced that the attack was carried out by the Syrian government.
They also claim Syria's permission to investigate came too late, possibly allowing government forces sufficient time to clear evidence in Ghouta. UK Foreign Secretary William Hague suggested that expectations should be "realistic" for what the UN team can achieve.
US President Obama and UK Prime Minister Cameron issued a joint warning on Saturday that they would consider a “serious response” if it emerged that the Syrian government had indeed been behind the attack.
Russia, a major ally of Syria, has been more skeptical about the Assad regime's culpability, saying that the attack was carried out by opposition fighters to increase international pressure on Bashar al-Assad's government.
The US and its allies deem such a possibility to be unlikely as they believe that rebels do not have access to nerve gas agents or the missiles necessary to use them in such an attack.
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