Russian Foreign Minister Sergei Lavrov has said a reported chemical attack in Syria was staged by foreign agents.
The US and France say they have proof it took place and, alongside the UK, are considering military retaliation.
Russia, which has military forces deployed in Syria in support of the government, has warned that US air strikes risk starting a war.
The UN’s secretary general has said the Middle East is “in chaos” and the Cold War is “back with a vengeance”. Antonio Guterres was speaking to a special meeting of the UN Security Council, called by Russia.
Independent chemical weapons inspectors are expected to arrive in the area of the alleged attack on Saturday.
During a press briefing on Friday, Mr Lavrov said he had “irrefutable evidence” that the attack was staged as part of a “Russophobic campaign” led by one country, which he did not name.
The White House says it is continuing to assess intelligence and talk to its allies about how to respond.
A delegation from the Organisation for the Prohibition of Chemical Weapons (OPCW) will start its investigations in Syria’s Eastern Ghouta region on Saturday but few details are expected to be released about its movements for safety reasons.
The suspected attack, denied by Syrian President Bashar al-Assad’s government, was carried out in the Eastern Ghouta town of Douma on Saturday, reportedly killing dozens of people.
Control over the town has since passed from rebels to the Syrian and Russian military authorities.
The Violations Documentation Center (VDC), a Syrian opposition network which records alleged violations of international law in Syria, said bodies were found foaming at the mouth, and with discoloured skin and burns to the eyes.
On Thursday, unnamed US officials told NBC News they had obtained blood and urine samples from victims which had tested positive for chlorine and a nerve agent.
The US ambassador to the UN, Nikki Hayley, told the network: “We definitely have enough proof but now we just have to be thoughtful in our action.”
French President Emmanuel Macron also said he had “proof” that the Syrian government had attacked Douma with chemical weapons but did not give further details.
