A team of UN weapons inspectors has arrived in the Syrian capital Damascus to assess whether chemical weapons have been used in the conflict.
Both the rebels and government forces have accused each other of using chemical weapons, with the town of Khan al-Assal at the centre of allegations.
Some 26 people were killed in attacks in the northern town in March.
The UN mission was delayed because of differences with the Syrian government over the scope of the investigation.
However, the Syrian government agreed last month to allow inspectors in to examine three locations including Khan al-Assal.
The 20-member team of UN weapons inspectors and public health specialists checked into the Four Seasons hotel in Damascus on Sunday, but declined to speak to reporters on their arrival.
Deputy Foreign Minister Faisal Mekdad told the Associated Press news agency that Syria will "fully cooperate" with the team.
The mandate of the investigating team, led by Swedish arms expert Ake Sellstrom, is limited to reporting on whether chemical weapons were actually used and which ones, but it will not determine responsibility for any attacks.
Experts warn that with so much time lost, it will be difficult for the team to be able to establish beyond a doubt that such weapons were used.
While Khan al-Assal has been identified as one of the locations to be investigated, the remaining two sites have not yet been specified.
The town has fallen under rebel control since the reported incident there in March, further complicating the situation, says the BBC's Jim Muir.
The issue of chemical weapons use is highly charged, he adds, especially since US President Barack Obama warned that their use by the regime would cross a "red line" and be a "game changer".
The possibility of President Bashar al-Assad using Syria's chemical weapons stock or rebels obtaining some of the stockpiles is one of the factors that has most worried Western observers of the conflict.
The UN says it has received up to 13 reports of chemical weapons use in Syria - one from the Damascus government about the events at Khan al-Assal, with the rest mainly from the UK, France and USA.
Syria is one of seven countries that have not joined the 1997 convention banning chemical weapons.
Syria is widely believed to possess large undeclared stockpiles of mustard gas and sarin nerve agent.