A senior US official said that Syrian president Bashar Al Assad and his backers will not be allowed to pursue a military campaign and seek a political solution at the same time.
The remarks came on Friday, a day after the US concluded that the Assad regime had again used chemical weapons this year, soon after starting an offensive to reclaim the rebel-held north-western province of Idlib.
Ambassador James Jeffrey, the US special envoy for Syria, said a constitutional committee agreed between the government in Damascus and the opposition meant it was time for Mr Al Assad to choose between war or peace negotiations.
“A military victory for Assad in this – or anybody else in this conflict and a political settlement are mutually exclusive,” said Mr Jeffrey.
“You have to decide and the time now is to decide, not only for Assad, but for those who support him. This is a message we've been passing on to the Russians with whom we are dealing very closely on this.”
Mr Jeffrey was speaking on the sidelines of the United Nations General Assembly. The UN confirmed on Monday agreement between the Syrian government and opposition on the constitutional committee.
The panel, which will have co-chairs and 150 members, is considered a tentative first step towards peace. It is due to start its work in the next four weeks, tasked with writing a new constitution that would shape the path to elections.
The UN-brokered process that led to the committee has taken almost two years of a now near-years-long civil war.
The Assad regime controls about 60 per cent of Syrian territory and its assault on Idlib is a major military step that has killed more than 1,100 civilians since late April and displaced 400,000 others, according to UN agencies and aid groups.
On Thursday, US Secretary of State Mike Pompeo said Syrian forces had used chlorine gas in a chemical attack in Latakia province, which borders Idlib. Although there were no fatalities, four people were left suffering the effects of chlorine.
International investigators say the Syrian regime has repeatedly used chemical weapons against civilian targets in the civil war.
The US and France had earlier told of their suspicions of a chemical attack but held off making a formal determination, saying more research was needed.
Along with Britain, the US and France has said that the use of chemical weapons could result in military action against Mr Al Assad's forces.
Mr Jeffrey said proportionality would govern any US response.
“Just because this time we didn't use a military option, that military option remains very much on the table in response to a chemical weapons attack,” he said.
"Again, the circumstances will determine ... not whether we will react, but how we react."
UN Secretary General Antonio Guterres, meanwhile, met Syria’s Deputy Prime Minister and Foreign Minister Walid Al Mouallem, also in New York for the General Assembly.
“The secretary general underscored the importance of launching the recently agreed credible and inclusive constitutional committee and making progress on confidence-building measures,” a spokesman for Mr Guterres said.
