Independent investigators have been prevented by Syrian and Russian officials from reaching the scene of an alleged chemical attack near the Syrian capital, an official said.
The barrier for the inspectors comes days after the US, UK and France bombarded sites they said were linked to Syria's chemical weapons programme.
The lack of access to the town of Douma by investigators from the watchdog group, the Organisation for the Prohibition of Chemical Weapons (OPCW), leaves questions about the April 7 attack unanswered.
OPCW director-general Ahmet Uzumcu said Syrian and Russian officials cited "pending security issues" in keeping its inspectors from reaching Douma.
"The team has not yet deployed to Douma," two days after arriving in Syria, Uzumcu told an executive council of the OPCW in The Hague.
Syrian authorities are offering 22 people to interview as witnesses instead, he said, adding that he hoped "all necessary arrangements will be made ... to allow the team to deploy to Douma as soon as possible".
Early on Tuesday, Syria's government-run Central Military media reported a missile attack on the Shayrat air base in Homs province.
It said Syrian air defences shot down most of the six missiles fired at the base. It also reported a separate air strike on the Dumayr air base near Damascus.
It did not elaborate or say who carried out the air strikes. A Pentagon spokeswoman said there was no US military activity in the area.
Earlier this month, four Iranian military personnel were killed in an airstrike on Syria's T4 air base in Homs. Syria and its allies blamed Israel for that attack. Israel did not confirm or deny mounting the raid.
Meanwhile, the US and France say they have evidence that poison gas was used in Douma, east of Damascus, killing dozens of people, and that President Bashar Assad's military was behind it, but they have made none of that evidence public. Syria and its ally Russia deny any such attack took place.
Deputy Russian foreign minister Sergei Ryabkov blamed the Western air strikes carried out early on Saturday for holding up a mission by the OPCW team to Douma. He said the inspectors cannot access the site because it needs permission from UN security experts.
Mr Ryabkov's remarks could indicate a possible attempt to bog down the team, even though both Russia and the Syrian government have welcomed the OPCW visit.
He told reporters in Moscow that what is hampering a speedy resolution of the visit to Douma was "the consequences of the illegal, unlawful military action", a reference to the punitive airstrikes.
The OPCW team arrived in Syria shortly before the air strikes and met with Syrian officials. Government forces and Russian troops have been deployed in Douma, which is now controlled by the Syrian government.
Syrian opposition and activists have criticised the Russia deployment in the town, saying that evidence of chemical weapons' use might no longer be found.
Russian foreign minister Sergey Lavrov denied that Russia interfered with any evidence.
"I can guarantee that Russia has not tampered with the site," Mr Lavrov told the BBC in an interview.
The Kremlin also denied that Russia was not allowing the OPCW mission in.
The OPCW is holding an emergency meeting in The Hague to discuss the suspected chemical attack in Douma.
At least 40 people are believed to have died in the attack on Douma, which until Saturday was the last rebel-held town near Damascus.
The Russian military later said that it will help secure the visit of international chemical weapons inspectors to the site of the alleged attack.
Maj Gen Yuri Yevtushenko, of the military's Reconciliation Centre in Syria, said Russian military police were ready to help protect the experts on their visit to Douma, outside Damascus.
Igor Kirillov, a top Russian chemical weapons protection expert, said experts from the Organisation for the Prohibition of Chemical Weapons are set to visit Douma on Wednesday.