UN inspectors tasked with investigating whether chemical weapons have been used in the Syrian conflict arrived in Damascus on Sunday, an AFP journalist reported.
The UN team arrived at the Four Seasons hotel in the Syrian capital to begin their hard-won mission which UN officials have said will last two weeks.
The mission had been repeatedly delayed earlier this year amid differences with President Bashar al-Assad's regime over the scope of the probe into the alleged use of chemical arms in the 29-month civil war.
But the Syrian government insisted on Thursday that it had nothing to hide.
Both the government and the rebels fighting to overthrow Assad accuse each other of using chemical weapons.
Led by Swedish arms expert Aake Sellstroem, the team is expected to investigate Khan al-Assal, near Aleppo, where the government says rebels used chemical weapons on March 19, killing at least 26 people, including 16 Syrian soldiers.
The opposition says government forces carried out the attack.
Khan al-Assal has since fallen into rebel control, raising questions about how the UN team might reach the site.
The Syrian opposition says the investigators can have full access to sites under its control where chemical weapons are alleged to have been used.
Damascus had called for a UN probe in March but insisted it focus solely on Khan al-Assal.
UN Secretary General Ban Ki-moon, however, pressed for broader access and investigation of other sites as well.
The United Nations has not yet identified two other sites where investigations are to be carried out.
"Our goal remains a fully independent and impartial inquiry," said a UN statement.
"The secretary general believes that an effective mechanism to investigate allegations of the use of chemical weapons can serve as an important deterrent against their employment," it added.
"The overwhelming support of the international community for this investigation makes clear that the use of chemical weapons by any side under any circumstances would constitute an outrageous crime."
US President Barack Obama has warned the Damascus regime that any use of chemical weapons against civilians would cross a "red line" and constitute a "game-changer".