Pressure mounted on Damascus to allow a UN probe of chemical attacks, with French President Francois Hollande saying evidence indicated the regime in war-ravaged Syria was to blame and Israel demanding action against its neighbour.
The Al-Nusra Front, a fierce Al-Qaeda-linked group fighting the regime of Bashar al-Assad, vowed revenge against villages of the Syrian president's minority Alawite community.
Doctors Without Borders said 355 people died this week of "neurotoxic" symptoms, after Syria's opposition claimed regime forces unleashed chemicals east and southwest of Damascus on Wednesday causing more than 1,300 deaths.
If confirmed, it would be the deadliest use of chemical agents since late Iraqi dictator Saddam Hussein gassed Iranian troops and Kurdish rebels in the 1980s.
US President Barack Obama said a year ago that the use of chemical weapons by Assad's forces was a "red line" that could trigger Western intervention.
On Sunday a strident warning came from Washington's archfoe Iran.
"If the United States crosses this red line, there will be harsh consequences for the White House," armed forces deputy chief of staff Massoud Jazayeri said.
