A team of UN inspectors is due to visit the site of a suspected chemical weapons attack carried out on Wednesday near the Syrian capital Damascus, BBC reports.
The visit is going ahead after Syria's government agreed to a ceasefire.
However its decision to allow monitors to access the site has been met with scepticism by Western governments.
Meanwhile, Syrian President Bashar al-Assad has said any military intervention by the US would end in "failure".
He told the Russian newspaper Izvestia that Western leaders would not succeed in turning Syria into a puppet of the West.
The US says there is little doubt Syrian forces used chemical weapons in the attack, which reportedly killed more than 300 people. The government blames rebels.
A year ago, US President Barack Obama said the use of chemical weapons by the Syrian government would be "a red line" that could trigger US military action.
Washington has bolstered its naval presence in the eastern Mediterranean as it considers its options.
As reported earlier on 21 August, the Syrian opposition said the authorities had used toxic agents by delivering strikes on Damascus’ suburbs. The victims reportedly exceeded 300 people. Damascus named reports as the slander, which undermined the work of U.N. inspectors in Syria.