Air raids and shelling of southern Idlib and northern Hama provinces escalated over the past week after Moscow and Tehran rejected a Turkish ceasefire proposal at a trilateral summit held in the Iranian capital on September 7.
Howitzers and armored cars were also headed for the border with Syria's Idlib province on Monday, according to state-run Anadolu Agency, and the army reinforced 12 outposts on Syrian territory where Turkish soldiers along with Russians and Iranians monitor flare-ups of violence, according to reports.
Last week the French ambassador, whose country also vowed to strike Syria if what it deems credible chemical allegations emerge, said during a U.N. Security Council meeting on Idlib: "Syria is once again at the edge of an abyss".
"We've tried to convey the message in recent days that if there's a third use of chemical weapons, the response will be much stronger", national security adviser John Bolton said while fielding questions after a policy speech.
"Getting smuggled across is nearly impossible, and there have been several cases of displaced people being shot dead while trying to enter Turkey illegally", Rifai said.
If chemical weapons are used, National Security Adviser John Bolton promised the USA would deliver a counterattack that's even more severe than the two previous assaults authorized by Trump. "That's something we're monitoring very closely", OCHA spokesman David Swanson said.
We want to tell you: if you sell Idlib or other opposition-held areas, we're going to buy Reyhanli from you. Syrian and Russian officials - key allies in Syria's long conflict - appear to be preparing for an all-out assault to retake the area for good.
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The offensive came just hours after more than 60 air raids killed at least four civilians in southern Idlib on Saturday.
Idlib and nearby areas are largely controlled by Hayat Tahrir al-Sham, an alliance led by Al-Qaeda's former Syrian affiliate, as well as rival rebels.
It is home to some three million people - around half of them displaced from other parts of the country, according to the United Nations.
Russian Federation has said it wants all militants to be pushed out of Idlib and that it avoids civilians and targets only radical al Qaeda-inspired groups.
Syria's conflict has killed more than 350,000 people and forced millions more out of their homes, but the United Nations has warned a full-blown attack on Idlib could bring unprecedented suffering. The OCHA chief, Mark Lowcock, warned that this risked provoking the worst humanitarian catastrophe of the 21st Century.
Turkey, fearing an influx of refugees in the event of a major assault, has repeatedly warned against a "massacre" in this last stronghold of the Syrian opposition.
He called instead for an "international counterterrorism operation" in cooperation with Turkish-backed opposition factions to weed out groups like Hayat Tahrir al-Sham that have been blacklisted by the global community.