With the United Nations warning of a potential "bloodbath" in Syria's Idlib province, the United States warned President Bashar al-Assad on Tuesday it would respond "swiftly and appropriately" if he uses chemical weapons against his own people.
As Syrian troops massed near the northwestern region of nearly three million people, the last major rebel stronghold, renewed Russian airstrikes killed at least nine civilians, according to a violence monitor.
Nikki Haley, the US ambassador to the United Nations, said the UN Security Council would meet on Friday to discuss the precarious situation in the province.
Rebel-backer Turkey has held several rounds of talks with regime ally Russia aimed at averting an assault on Idlib, but Moscow dubbed the region a "pocket of terrorism."
The UN's Syria peace envoy Staffan de Mistura said Russia and Turkey held "the key for (a) soft solution to the Idlib issue" and urged them to help avert a humanitarian disaster.
De Mistura pointed to press reports indicating that Syria has set a September 10 deadline for finding a solution before it begins an all-out offensive on the province.
"Let's try to avoid that the last probably major battle of the Syrian territorial conflict... ends in a bloodbath," De Mistura told reporters in Geneva.
US President Donald Trump warned Assad on Monday not to "recklessly attack" Idlib and the White House strongly cautioned him Tuesday against using chemical weapons.
"Such an attack would be a reckless escalation of an already tragic conflict and would risk the lives of hundreds of thousands of people," said White House spokeswoman Sarah Sanders, warning the US and its allies "will respond swiftly and appropriately."
Haley's own warning focused more explicitly on the prospect of a chemical attack.
"If they want to continue to go the route of taking over Syria, they can do that, but they cannot do it with chemical weapons," she said.
- 'Pocket of terrorism' -
De Mistura urged Russian President Vladimir Putin and his Turkish counterpart Recep Tayyip Erdogan to urgently speak on the phone even before they are set to meet with their Iranian counterpart in Tehran on Friday.
His appeal came after Russian warplanes resumed airstrikes on Idlib after a 22-day pause.
The Britain-based Syrian Observatory for Human Rights, which relies on sources in Syria for its reports, said at least nine civilians, including five children from the same family, were killed in the raids, while 10 people were wounded.
Seized from government forces in 2015, Idlib and adjacent areas form the last major chunk of territory still in rebel hands.
Many of the residents are rebels and civilians who were bussed out of other areas as they came back under Syrian regime control.
Tuesday's bombardment hit several areas held by the jihadist-led Hayat Tahrir al-Sham alliance, among them the large town of Jisr al-Shughur, but also areas held by rival Turkish-backed rebels, including the town of Ariha.
The Syrian military has been deploying reinforcements to the zone for more than a month, and Russia has stepped up its rhetoric.
"We know that the Syrian armed forces are getting ready to solve this problem," Putin's spokesman Dmitry Peskov said, calling Idlib a "pocket of terrorism."
Moscow has been carrying out strikes in Syria since September 2015, using aircraft at the Hmeimim base in Latakia province.
It accuses rebels in Idlib of attacking Hmeimim with weaponized drones and insists jihadists in the province must be eliminated.
- 'Appeal for sanity' -
De Mistura said there were an estimated 10,000 fighters with UN-designated terror groups currently in Idlib.
But he stressed that there are some 2.9 million civilians in the province, including around one million children, who "are not terrorists."
"There are many more babies than terrorists in Idlib," said Jan Egeland, head of the UN's humanitarian taskforce for Syria. "We appeal for sanity."
Amid a diplomatic frenzy to try to avoid a full-scale assault on Idlib, French Foreign Minister Jean-Yves Le Drian told AFP Tuesday "it is important today that the path to negotiations be open and not that toward confrontation."
De Mistura, whose years of efforts to push forward a Syrian peace deal have achieved no breakthroughs, insisted that he would pursue his efforts to set up a committee to write a new Syrian constitution.
The UN envoy said he did not plan to postpone two rounds of consultations next week with high-level representatives of a range of countries with influence on different sides in the conflict.
"It is going to be a moment of truth," he said.
More than 350,000 people have been killed and millions displaced since Syria's war started in 2011 with the brutal repression of anti-government protests.