UN Secretary General: 'Cold War is back with vengeance'
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.
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