Britain, France and the United States called Wednesday for a United Nations inquiry into accusations by the Syrian government and the opposition of chemical weapons attacks.
But Russia, President Bashar al-Assad's last major ally, insisted at a UN Security Council meeting that only the government's allegations should be investigated.
Syria has asked the United Nations to order an "independent" investigation into its claim that opposition rebels used chemical weapons in an attack at Khan al-Assal near Aleppo on Tuesday.
The opposition has accused the government of staging the Aleppo attack and another with chemical weapons at Atayba near Damascus.
UN leader Ban Ki-Moon gave no immediate sign as to how he would respond to the calls for an investigation. UN spokesman Martin Nesirky said a written request has been from the Damascus government "and it is being studied."
Divisions over the two year-old Syria conflict again erupted at a Security Council meeting where France raised the allegations.
French UN ambassador Gerard Araud said there was no proof yet of the use of chemical weapons by any side.
He told reporters that a majority of the 15 members of the council had wanted to send a letter to UN Secretary General Ban Ki-Moon calling for an investigation into "all allegations of the use of chemical weapons."
Signup to our newsletter and follow us on Facebook and Twitter!
If proved "it would be a very serious violation of international law," Araud added.
Britain and the United States back the calls for an investigation into all allegations. The Western countries said they would send a letter to Ban seeking the signatures of as many council members as possible.
"The facts need to be clarified," said Britain's deputy UN ambassador Philip Parham.
"If chemical weapons have been used, this would be abhorrent, it would be very grave, it would warrant a serious response by the international community."
Russia's UN ambassador Vitaly Churkin said however the western initiative was an attempt "to delay" any investigation into claims of weapons use in Aleppo. He said France had sought an investigation into "rumours".
"Instead of launching those propaganda balloons I think that its much better to get our focus right and I hope this is what the secretary general is doing," Churkin told reporters.
Syria's UN envoy Bashar Jaafari said the accusation of government use of banned weapons "was set up on purpose to torpedo the investigation into the real use of chemical weapons in Aleppo yesterday."
However British envoy Parham countered that the way Syria has "framed the request pre-judges the outcome of the investigation by alleging it is the opposition which is responsible."
The Syria war has torn apart the Security Council.
Russia and China have blocked three council resolutions, proposed by the west, which sought to increase pressure on Assad's government.