But Syria's UN ambassador refuted regime media reports that chemical inspectors had entered Douma, saying that UN security experts were preparing the way for the OPCW inspectors to look into alleged attack.
Syrian media said international chemical weapons inspectors on Tuesday entered the town of Douma, where an alleged poison gas attack was carried out earlier this month, but a Syrian diplomat said later that a UN security team only visited the Damascus suburb.
Earlier state media had reported that the fact-finding team from the Organization for the Prohibition of Chemical Weapons (OPCW) entered Douma town in eastern Ghouta area in Damascus countryside on Tuesday.
The reported attack that the inspectors are investigating led to Western air strikes against the Syrian regime over the weekend.
Syria's UN ambassador, Bashar Jaafari, told the Security Council that the UN team went to Douma to decide whether investigators from the international chemical weapons watchdog could safely visit the site. If the team decided "the situation is sound," the fact-finding mission from the OPCW would start work there on Wednesday, Jaafari said
The security team's entry into Douma came 10 days after the alleged attack, raising concerns that any evidence the inspectors find could be useless.
On Monday, OPCW Director-General Ahmet Uzumcu said Syrian and Russian authorities had blocked its inspectors from going to Douma and instead offered them, 22 people, to interview as witnesses. The team arrived in the Syrian capital of Damascus on Saturday.
Journalists in Damascus were prevented by regime minders from contacting the OPCW inspectors, and The Hague-based organisation refused to comment on "operational details regarding the Douma deployment."
UN Secretary-General Antonio Guterres said it was the "obligation of the Syrian government to provide all the conditions for (OPCW inspectors) to work without any restrictions."
British Prime Minister Theresa May accused the Syrian regime and its ally Russia of trying to cover up evidence and obstruct the investigation.
The OPCW is investigating reports that regime forces gassed sites in Douma on April 7, when the town was still held by rebels and home to tens of thousands of people – residents and others who were displaced by fighting elsewhere.
Syrian activists said more than 40 people were killed in the alleged attack.
Less than two days later, the Army of Islam rebels surrendered the town, which was the last stronghold in the once-rebellious eastern Ghouta region at the doorstep of Damascus.
The US and France say they have evidence that Syrian regime leader Bashar al Assad's military was behind the poison gas attack, but they have made none of that evidence public. On Saturday, the US, France and Britain bombarded sites they said were linked to Syria's chemical weapons programme.
Journalists were allowed access to Douma on Monday. The Associated Press spoke to survivors and witnesses who described being hit by gas, fainting and discovering their relatives had died, with foam bubbling around their mouths.
The Syrian regime and Russia have denied using chemical weapons and accused rebels, with Western backing, of using such weapons or faking such an attack.
Russian Foreign Ministry spokeswoman Maria Zakharova dismissed allegations that Russia was trying to hamper the OPCW inspectors, saying Moscow strongly supported their mission to Douma.
Alexander Rodionov of the Russian military's chemical weapons protection unit said its experts found chlorine and components for producing mustard gas at a rebel laboratory in Douma. He said the canister with chlorine was similar to one shown in images released by activists in the days after April 7.
Russian President Vladimir Putin told German Chancellor Angela Merkel that the Western strikes had violated international law and set back the peace process, the Kremlin said.
But French President Emmanual Macron defended the military action in an impassioned and at times angry speech to the European parliament.
He spoke of the "outrage by images we've seen of children, women who died of a chlorine attack."
"Do we sit back, do we defend (human) rights by saying, 'Rights are for us, principles are for us, and realities are for others?' No, no!" the French leader said. "Three countries have intervened, and let me be quite frank: this is for the honour of the international community."
The alleged attack and subsequent military response underscored the challenge that the seven-year conflict in Syria poses for the West. President Donald Trump wants to remove US troops from Syria, where it has been fighting Daesh terrorists.