UN disarmament chief Angela Kane has arrived in Damascus to push the Syrian government for access to the alleged site of a chemical weapons attack.
It comes as France joined the UK in accusing Bashar al-Assad's forces of carrying out the attack in the capital's eastern suburbs on Wednesday.
US President Obama has said he is weighing his options amid claims that hundreds of civilians were killed.
State media on Saturday said chemical agents were found in rebel-held areas.
Syrian television said soldiers had "suffocated" as they tried to enter Jobar - one of the towns in the Ghouta district around Damascus reportedly attacked on Wednesday.
The foreign ministries of Russia and Iran, both allies of Syria, have separately accused rebels of using chemical weapons.
Damascus has called the allegations "illogical and fabricated".
Syria's opposition said hundreds of people died in a night-time government assault in Ghouta on Wednesday.
Unverified footage shows civilians - many of them children - dead or suffering from what appeared to be horrific symptoms consistent with a chemical attack.
The allegations have caused international outrage.
France's Foreign Minister Laurent Fabius said on Saturday that "all the information at our disposal converge to indicate that there was a chemical massacre near Damascus and that the [regime of Bashar al-Assad] is responsible".
A day earlier, UK Foreign Secretary William Hague said he believed "this is a chemical attack by the Assad regime" and it was "not something that a humane or civilised world can ignore".
UN Secretary General Ban Ki-moon said he was determined to "conduct a thorough, impartial and prompt investigation" into the events.
He has sent Angela Kane to press the Syrian authorities to allow a team of 20 experts on chemical weapons - already in Damascus - to investigate the claims.
The US, meanwhile, is facing rising pressure to intervene.
President Obama met his national security team on Saturday to discuss possible next steps in Syria.
US defence officials have said navy commanders are keeping a fourth warship in the eastern Mediterranean Sea.
The officials stressed that the US Navy had received no orders to prepare for military action.
Mr Obama told CNN that Wednesday's attack was a "big event of grave concern", but added that the US was still seeking confirmation that chemical weapons had been used and warned against a knee-jerk reaction.
Last year, he said the use of chemical weapons in Syria would cross a "red line" and force a tough US response.
Russia's foreign ministry said Moscow had urged President Assad to co-operate with UN inspectors, but questioned the opposition's willingness to provide "secure, safe access of the [UN] mission to the location of the incident".
It also said there was evidence that "this criminal act was clearly provocative", referring to "reports circulating on the internet" about the video footage and "accusations against government troops being posted several hours before the so-called attack".
The UN says more than 100,000 people have been killed since the uprising against President Assad began more than two years ago.
'Chemical attack': What we know
- 01:15: 21 August (10:15 GMT 20 Aug): Facebook pages of Syrian opposition report heavy fighting in rebel-held districts of Ghouta, the agricultural belt in eastern Damascus
- 02:45: Opposition posts Facebook report of "chemical shelling" in Ein Tarma area of Ghouta
- 02:47: Second opposition report says chemical weapons used in Zamalka area of Ghouta
- Unverified video footage shows people being treated on pavements in the dark and in a makeshift hospital
- Reports say chemical weapons were used in Ghouta towns of Irbin, Jobar, Zamalka and Ein Tarma as well as in Muadhamiya to the west, but this is not confirmed
- Syrian government acknowledges military offensive in the Ghouta area but denies chemical weapons use