The chief of the Organisation for the Prohibition of Chemical Weapons "does not know" when a fact-finding mission will be able to travel safely to Douma in Syria.
OPCW Director-General Ahmet Uzumcu said the departure of a team of inspectors was delayed after an incident in which an advance UN security detail performing reconnaissance in Douma was forced to withdraw after being fired upon.
The UN security detail had made arrangements with the Syrian authorities to be escorted to a certain point, before Russian military police took over the escort.
The Syrian military is unable to enter Douma under an agreement reached to allow the evacuation of the area, Mr Uzumcu said.
When UN security forces arrived at the city yesterday as part of a reconnaissance visit, a large crowd gathered.
When they went to look at one potential site where chemical weapons were possibly used, they were subjected to gunfire and light shelling and withdrew, he added.
The fact-finding mission will be unable to depart until it is given a green light for departure by the UN security detail.
Earlier, Syria’s UN ambassador said the security team entered Douma yesterday to determine whether OPCW experts can deploy there today, Ambassador Bashar Jaafari said.
"If this United Nations security team decides that the situation is sound in Douma then the fact-finding mission will begin its work in Douma," Mr Jaafari told the Security Council.
The Syrian state news agency earlier reported that the international OPCW team had entered Douma to begin its investigation of whether chemical agents were used as a weapon.
The ambassador stressed that the "Syrian government did all that it can do to facilitate the work of this mission" but that it was up to the United Nations and the OPCW to decide whether to deploy, based on security considerations.
The inspectors arrived in Damascus on Saturday, when Britain, France and the United States launched military strikes against what they said were targets linked to Syria's chemical weapons programme.
The suspected 7 April gas attack on Douma, near Damascus, reportedly left more than 40 people dead and was blamed by Western powers on Syrian President Bashar al-Assad.
The OPCW team will seek evidence from soil samples, interviews with witnesses, blood, urine or tissue samples from victims and weapon parts. But, more than a week after the suspected attack, hard evidence might be hard to trace.
Douma was the last town to hold out in the besieged Eastern Ghouta enclave, the last big rebel bastion near the capital Damascus. Eastern Ghouta was captured by a government advance over the past two months.
The Syrian "White Helmets" rescue organisation, which operates in rebel-held areas, has pinpointed for the OPCW team the places where victims of the suspected attack are buried.
Douma hospital workers who stayed in the town after the army recaptured it have said that none of the people injured on the night of the attack were exposed to chemical weapons.
Medical charities operating in opposition-held parts of Syria have dismissed those statements as propaganda, given under duress now that government forces control the town.