As international chemical weapons experts travelled to Syria to investigate a suspected gas attack by government forces which killed dozens of people, French President Emmanuel Macron spoke to Russia's Vladimir Putin and expressed concern about a worsening situation.
Two days ago US President Donald Trump warned that missiles "will be coming" in response to the gas attack on Douma, a town near Damascus which had been held by rebels until this month.
Russia, the main ally of Syrian President Bashar al-Assad, says there is no evidence of a chemical attack in Douma and has warned the United States and its allies against conducting any military strike.
But US Ambassador to the United Nations, Nikki Haley, said on Friday that Washington estimated Assad's forces had used chemical weapons at least 50 times during the seven-year-long Syrian conflict.
"Our President has not yet made a decision about possible action in Syria. But should the United States and our allies decide to act in Syria, it will be in defense of a principle on which we all agree," Haley told the UN Security Council.
While Trump himself remained silent on Syria on Friday, Macron - who has said France has proof that Assad's government used chemical weapons in the attack in Douma - spoke by phone with Putin.
The Elysee palace said in a statement that Macron had also expressed regret about Russia's use of its Security Council veto on the attack. Striking a conciliatory note, it said: "The President of the Republic called for dialogue with Russia to be maintained and stepped up to bring peace and stability back to Syria."
But this was balanced by a warning from France's ambassador to the UN, Francois Delattre, who told the Security Council that the Syrian government's decision to use chemical weapons again meant they had "reached a point of no return".
The world must provide a "robust, united and steadfast response", he said.
Since 2015 France has carried out air strikes against Islamic State in Syria as part of allied forces linked to the US-led coalition, conducting about 5 percent of total coalition air missions.
There was no word from Putin himself, though Russian Foreign Minister Sergei Lavrov said Moscow was in contact with Washington to discuss an atmosphere which he described as alarming.
"God forbid anything adventurous will be done in Syria on the lines of the Libyan and Iraqi experience," he told a news conference, referring to Western interventions elsewhere in the region.
"Even non-significant incidents would lead to new waves of migrants to Europe and to other consequences, which neither we nor our European neighbours need," Lavrov said.
Russian Deputy Prime Minister Arkady Dvorkovich took an apparent swipe at Trump's tweets.
"We cannot depend on what someone on the other side of the ocean takes into his head in the morning. We cannot take such risks," he said at a forum.
Vassily Nebenzia, Moscow's UN ambassador said he "cannot exclude" war between the United States and Russia.
"The immediate priority is to avert the danger of war," he told reporters. "We hope there will be no point of no return."
Sheikh Naim Qassem, deputy leader of the Iranian-backed Hezbollah movement, told Lebanese daily al-Joumhouria: "The conditions do not point to a total war happening ... unless Trump and (Israeli leader Benjamin) Netanyahu completely lose their minds."
US allies have offered strong words of support for Washington but no clear military plans have yet emerged.
British Prime Minister Theresa May won backing from her senior ministers on Thursday to take unspecified action with the United States and France to deter further use of chemical weapons by Syria.
Some national leaders appeared anxious to avert a US-Russian showdown.
Tayyip Erdogan, president of Syria's neighbour Turkey which is also in NATO, said on Friday he had spoken by phone with Trump and Putin and told both that increasing tension in the region was not right.
A first team of experts from global watchdog, the Organisation for the Prohibition of Chemical Weapons, has arrived in Syria, UN Secretary-General Antonio Guterres said. They are expected to start their investigations into the Douma incident on Saturday, the Netherlands-based agency said.
In Geneva, UN war crimes investigators condemned on Friday the suspected use of chemical weapons in Douma and called for evidence to be preserved with a view to future prosecutions.
Assad tightens grip
Trump himself appeared on Thursday to cast doubt on at least the timing of any US-led military action, tweeting: "Never said when an attack on Syria would take place. Could be very soon or not so soon at all!"
He met his national security team later in the day and "no final decision has been made", the White House said in a statement.
"We are continuing to assess intelligence and are engaged in conversations with our partners and allies," it said.
Global stock markets have had a whipsaw week, largely fuelled by Trump's tendency to change his mind over top policy issues.
The capture of Douma has clinched a major victory for Assad, crushing what was once a centre of the insurgency near Damascus, and underlines his unassailable position in the war.
Assad, who is supported by Iranian-back fighters as well as the Russian air force, has cemented his control over most of the western, more heavily populated, part of the country. Rebels and jihadist insurgents are largely contained to two areas on Syria's northern and southern borders.
