UN weapons inspectors were due Monday to visit the site in Syria where chemical weapons were allegedly used and over which West has warned of consequences for the regime of President Bashar al-Assad, dpa reported.
The opposition said the August 21 bombardment by government forces using a poisonous gas left 1,300 people dead. The government has vehemently denied the claim.
Western nations have warned that the use of chemical weapons would be a trigger that could result in an armed reaction.
The United States said it was weighing its options on Syria and preparing for "all contingencies", while France warned that the use of such weapons would not go unpunished.
Syria allies Russia and Iran have meanwhile warned the US over the dangers of foreign incursions into Syria.
Russian Foreign Minister Sergei Lavrov in a telephone call with US Secretary of State John Kerry urged Washington "not to succumb to provocations" but to ensure that UN weapons inspectors can carry out an objective investigation, the Foreign Ministry in Moscow said.
Lavov also told Kerry that the US should not heed reports about a chemical weapons massacre near Damascus because the incident was most likely staged.
He warned that US intervention in Syria would "annihilate" the joint Russian-American efforts to help achieve peace.
The al-Assad regime has vehemently denied the allegations and accused the rebels of using chemical weapons.
"This is nonsense: first make accusations, and only then collect evidence. And this is being done by a powerful country - the United States," al-Assad said in an interview with the Russian newspaper Izvestia.
Damascus said Sunday it had reached an agreement with Angela Kane, the UN high representative for disarmament affairs, to allow the UN team to visit the area.
The inspection "will address the recent allegations in the Damascus area and its success can have a deterrent effect on possible further use of chemical weapons in Syria and elsewhere," UN Secretary General Ban Ki-moon said in Seoul.
"And every hour counts. We cannot afford any more delays. We have all seen the horrifying images on our television screens and through social media. Clearly this was a major and terrible incident. We owe it to the families of the victims to act."
A 20-member UN team, led by Swedish chemical weapons expert Ake Sellstrom, have been in Damascus since August 18 to investigate alleged use of chemical weapons in three limited areas, in particular the March 19 attack in Khan al-Assal area, in the northern province of Aleppo.
Western and Arab military officials are to meet in Amman to review "all options" in response to the alleged chemical attack in Syria.
More than 100,000 people have been killed in Syria since an uprising against Assad's rule started in March 2011, the UN says.