The international community on Wednesday called for investigations into reports that troops loyal to Syrian President Bashar Assad had carried out a chemical weapon attack near Damascus.
Russia's Foreign Ministry called for a fair and professional investigation into the allegations.
Foreign Ministry Spokesman Alexander Lukashevich said circumstances around the reports, including the presence of UN inspectors in the country, suggested that the attack could be a provocation by the opposition.
"All this cannot but suggest that once again we are dealing with a pre-planned provocation. This is supported by the fact that the criminal act was committed near Damascus at the very moment when a mission of UN experts had successfully started their work of investigating allegations of the possible use of chemical weapons there," Lukashevich said in a statement.
Russia is a longtime ally and arms supplier to Assad.
Meanwhile, the head of the United Nations chemical weapons inspectors in Syria said reports of a nerve gas attack killing hundreds of people near Damascus should be investigated.
Britain, France and the United States will send a letter to UN Secretary-General Ban Ki-moon later on Wednesday to officially request a UN investigation into the alleged gas attack, a UN diplomat said on condition of anonymity.
Swedish scientist Ake Sellstrom told news agency TT that while he had only seen TV footage, the high number of casualties reported sounded suspicious.
"It sounds like something that should be looked into," he told TT by phone from Damascus. "It will depend on whether any UN member state goes to the secretary general and says we should look at this event. We are in place."
In response to the reports, Arab League Secretary General Nabil Elaraby called for United Nations inspectors to immediately investigate reports of the chemical attack, Egypt's state news agency said.
"The secretary general said in a statement he was surprised this deplorable crime would happen during the visit of a team of international investigators with the United Nations who are already tasked with investigating chemical weapons use," the official news agency MENA said.
"He called on the inspectors to head immediately to the eastern Ghouta (suburb of Damascus) to determine what happened."
Britain said it would raise the reported chemical weapons attack by forces loyal to Syrian President Bashar Assad at the United Nations Security Council and called on Damascus to give UN inspectors access to the site.
"I am deeply concerned by reports that hundreds of people, including children, have been killed in airstrikes and a chemical weapons attack on rebel-held areas near Damascus," British Foreign Secretary William Hague said in a statement.
Hague said reports of the attack, which has been denied outright by the Syrian government, remained uncorroborated and that Britain was urgently seeking more information.
"But it is clear that if they are verified, it would mark a shocking escalation in the use of chemical weapons in Syria," he said, adding that Britain would try to hold to account anyone who used chemical weapons or ordered their use.
"I call on the Syrian Government to allow immediate access to the area for the UN team currently investigating previous allegations of chemical weapons use. The UK will be raising this incident at the UN Security Council," said Hague.
Britain provides non-lethal assistance to rebels fighting to overthrow Assad, but has stopped short of sending them arms and has seen its diplomatic efforts to put pressure on Assad frustrated by Russia and China at the United Nations.
The United States is "deeply concerned" by reports that hundreds of Syrian civilians have been killed in Syria in a chemical attack perpetrated by the government of Bashar Assad, the White House said on Wednesday.
The US will thus formally request that the United Nations "urgently investigate this new allegation," and has called an urgent meeting of the UN Security Council.
French President Francois Hollande called on United Nations inspectors to visit the site of the alleged chemical attack as well.
"With regard to the information coming out of Syria, the president asks that the UN goes to the site," Najat Vallaud-Belkacem told a weekly news briefing.
Turkey also called on UN inspectors to look into the Syrian rebel reports and said it was monitoring the situation "with great concern".
"Light must immediately be shed on these claims and the United Nations mission that was formed to investigate chemical weapons claims in Syria should look into these claims and reveal its findings," the Foreign Ministry said in a statement.
"If these allegations are found to be true, it will be inevitable for the international community to take the necessary stance and give the necessary response to this savagery and crime against humanity," it said.
Ankara, once an Assad ally but now one of his fiercest critics, has long been concerned about the possible use of Syrian chemical weapons against its own people.
Saudi Arabia also responded to the Syrian rebel reports and called for an emergency UN Security Council meeting, Saudi Foreign Minister Prince Saud al-Faisal said.
"It is time for the Security Council to shoulder its responsibility and overcome differences between its members and restore the confidence of the international community by convening immediately to issue a clear and deterrent resolution that will put an end to this human crisis," Prince Saud said in a statement.