The United Nations has said experts are ready to visit Syria to investigate claims of chemical weapons use during the country's two-year civil war.
However, a deal has not yet been reached with the Syrian government on safety assurances.
UN spokesman Eduardo del Buey said: "The devil is in the details and the details are being worked out."
It has been nearly two weeks since the UN said the Syrian government had agreed to let a team of experts travel to three sites where chemical weapons are reported to have been used.
One, Khan al-Assal in Aleppo, is where the Syrian government said rebels used chemical weapons in March.
The other two locations to be visited have not yet been identified.
The UN said it has received 13 reports of possible chemical weapons use - one from Syria's government and the rest mainly from Britain, France and the United States.
The Syrian government and the opposition have accused each other of using chemical weapons, and both have denied it.
The UN inquiry, led by Swedish scientist Ake Sellstrom, will only try to establish whether chemical weapons were used, not who used them.
Mr Sellstrom’s team is made up of experts from the Organization for the Prohibition of Chemical Weapons and the World Health Organization.
Rebels seized Khan al-Assal from Syrian President Bashar al-Assad's forces last month.
The Syrian National Coalition, the rebels' leadership group, has written to UN Secretary-General Ban Ki-moon saying they were ready to cooperate with the chemical weapons inquiry and "welcome UN investigators into all territories under our control".
Syria is one of seven countries that has not joined the 1997 convention banning chemical weapons.
Western countries believe it has stockpiles of undeclared mustard gas, sarin and VX nerve agents.
The UN says more than 100,000 people have been killed in the Syrian conflict.
More than 1.9 million Syrians have fled the country and more than 4.2 million people have been internally displaced, according to the UN.
