US President Donald Trump has warned the Syrian government and its Russian and Iranian allies against "recklessly" attacking rebel-held Idlib province.
In a tweet he warned of "a grave humanitarian mistake" in which hundreds of thousands of people could be killed.
Syrian government forces are said to be preparing a huge offensive on the last major rebel stronghold in the country.
The UN says such a campaign could have disastrous consequences for thousands of civilians.
The US state department also warned on Monday that Washington would respond to any chemical attacks by the Syrian government or its allies.
US ambassador to the UN Nikki Haley tweeted: "All eyes on the actions of Assad, Russia, and Iran in Idlib. #NoChemicalWeapons"
What could happen in Idlib?
With rebels defeated in most of Syria, the offensive in the northern province could prove to be the last major battle of the Syrian civil war.
According to a UN estimate, there are about 30,000 al-Nusra and al-Qaeda jihadist fighters still holding out in Idlib.
Sources quoted by Reuters news agency said that President Bashar al-Assad was preparing a phased offensive.
Russian Foreign Minister Sergei Lavrov has said "terrorists" must be wiped out in Idlib, accusing them of using civilians as human shields.
Both he and his Syrian counterpart, Walid Muallem, have accused rebels of preparing to stage a chemical attack in Idlib in order to blame pro-government forces and draw new US military retaliation.
Kremlin spokesman Dmitry Peskov has said that recent naval drills off Syria were justified, adding that the "hotbed of terrorists" in Idlib must be tackled.
What has the UN said?
UN envoy to Syria Staffan de Mistura has warned of a "perfect storm" if the government goes ahead with its threatened offensive in Idlib.
He called for humanitarian corridors to be set up to allow civilians to be evacuated temporarily.
The jihadists must be defeated, Mr de Mistura said, but not at the expense of thousands of civilian lives.
"There is a perfect storm based on warnings, counter-warnings which is gathering around and due to the dilemma, which is a true dilemma on how to defeat terrorists in Idlib and at the same time avoid affecting a huge number of civilians," he said.
Credit: The BBC