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.
What proof is there of a chemical attack?
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.
A UN report last year found the Syrian government responsible for a deadly chemical attack on the rebel-held town of Khan Sheikhoun, in which more than 80 people died.
That attack was followed by a US cruise missile attack on a Syrian airbase.
How is the West responding this time?
In the UK, cabinet ministers agreed it was “highly likely” the government of Syrian President Bashar al-Assad was responsible for the alleged attack and said the use of chemical weapons must not “go unchallenged”.
During a phone call late on Thursday, UK Prime Minister Theresa May and US President Donald Trump agreed to “keep working closely” on the issue, Mrs May’s office said.
The US president has said Russian President Vladimir Putin bears responsibility for the alleged attack in Douma because of his support for the Syrian government.
On Wednesday he warned Russia that missiles were “coming” but later tweeted that he had “never said when”. It “could be very soon or not so soon at all”, he said.
What is Russia saying about the risk of war?
Russia has described the reports of a chemical attack as a “provocation” designed to justify Western intervention.
Moscow’s UN ambassador Vassily Nebenzia accused Washington of putting international peace at risk.
“The immediate priority is to avert the danger of war,” he said on Thursday.
Senior Russian figures, including the head of the military, have warned that US missiles will be shot down and their launch sites targeted if Russian personnel come under threat.
On Friday, Deputy Prime Minister Arkady Dvorkovich criticised Mr Trump’s rhetoric.
“We cannot depend on the mood of someone on the other side of the ocean when he wakes up,” the Tass news agency quoted him as saying.
After six weeks of heavy fighting and an estimated 1,700 civilian deaths in the Eastern Ghouta, rebels are now leaving the area.
The final evacuations of about 4,000 remaining Islamist fighters and civilians were taking place on Friday, according to the UK-based Syrian Observatory for Human Rights monitoring organisation.