At least 58 people, including 11 children, have been killed and dozens wounded in a suspected chemical attack in rebel-held Syrian town of Idlib, the Syrian Observatory for Human Rights says.
The monitoring group reported that strikes on Khan Sheikhoun by Syrian government or Russian jets had caused many people to choke.
Later, aircraft fired rockets at local clinics treating survivors, doctors and activists said.
A Syrian military source denied the government had used any such weapons.
Russia’s defense ministry meanwhile insisted it had not carried out any air strikes in the vicinity.
If confirmed, it would be one of the deadliest chemical attacks in Syria’s civil war.
The warplanes are reported to have attacked Khan Sheikhoun, about 30 miles south of Idlib, on April 4, when many people were asleep.
Hussein Kayal, a photographer for the pro-opposition Edlib Media Center (EMC), told the Associated Press that he was awoken by the sound of an explosion at about 06:30.
When he reached the scene, there was no smell, he said. He found people lying on the floor, unable to move and with constricted pupils, he added.
The Syrian Observatory (SOHR) quoted doctors as saying that they had been treating people with symptoms including fainting, vomiting and foaming at the mouth.
An AFP journalist saw a young girl, a woman and two elderly people dead at a hospital, all with foam still visible around their mouths.
The journalist also reported that the same facility was hit by a rocket on April 4, bringing down rubble on top of doctors treating the injured.
The source of the projectile was not clear, but the EMC and the opposition Local Co-ordination Committees network said warplanes had targeted several clinics.
The SOHR put the death toll at 58, including 11 children, but the head of a charity ambulance service in Idlib, Mohammed Rasoul, said that 67 people had been killed and that 300 were injured.
The pro-opposition Step news agency meanwhile said 100 had died.
Sarin inhibits the action of an enzyme, which deactivates signals that cause human nerve cells to fire. This blockage pushes nerves into a continual “on” state. The heart and other muscles – including those involved in breathing – spasm.
Sufficient exposure to Sarin can lead to death via asphyxiation within minutes.
The substance is almost impossible to detect because it is a clear, colorless and tasteless liquid that has no odor in its purest form.
In January 2016, the organization said blood samples taken from the victims of one unspecified attack showed victims had been exposed to Sarin or a Sarin-like substance.
A joint investigation with the UN concluded in October that Syrian government forces had used chlorine as a weapon at least three times between 2014 and 2015.
It also found ISIS militants had used the blister agent sulphur mustard.
Human Rights Watch also recently accused government helicopters of dropping bombs containing chlorine on rebel-held areas of Aleppo on at least eight occasions between November 17 and December 13, during the final stages of the battle for the city.
Idlib province, where the air strikes took place, is almost entirely controlled by a rebel alliance and the al-Qaeda-linked jihadist group, Hayat Tahrir al-Sham.
The region, home to 900,000 displaced people, is regularly targeted by the government and its ally Russia, as well as the US-led coalition against ISIS.
There was no immediate comment from the government, but a Syrian military source told Reuters that it “does not and has not” used chemical weapons.
A Russian military helicopter has been shot down by rebels in Idlib, northern Syria, killing five people on board, Russia has said.
The Mi-8 chopper was carrying three crew and two officers, Russia’s defense ministry said in a statement.
The aircraft was returning from delivering humanitarian aid to the besieged city of Aleppo, the statement added.
It is not clear which group brought the helicopter down.
An alliance of rebel groups, including hardline jihadist factions, is the dominant power in Idlib.
Russia has previously, though seldom, lost aircraft since it launched operations in support of the Syrian government at the end of September 2015.
In July 2016, two Russian pilots were killed when their helicopter was shot down east of Palmyra by ISIS.
In November 2015, the pilot of a Russian Su-24 fighter plane was killed when the aircraft was shot down by Turkey on its border with Syria.
A Russian marine sent on a mission to rescue the pilot was also killed when his helicopter was shot down.
Pictures on social media purportedly of the latest Russian helicopter downing showed burning wreckage and bodies, with armed men milling around.
Footage showed at least one body being dragged away.
Russia is a key backer of Syrian President Bashar al-Assad and is supporting pro-government forces with air strikes on rebels.
Government forces cut off rebel-held eastern parts of Aleppo last month.
Russia and Syria announced the opening of what they called humanitarian corridors for civilians and rebels wanting to surrender, but few people are reported to have used them, fearing they would be targeted.
Russian President Vladimir Putin has defended his country’s military operations in Syria, saying the aim is to “stabilize the legitimate authority” of Syrian President Bashar al-Assad.
Vladimir Putin told Russian state TV Rossiya-1 Moscow also wanted to “create conditions for a political compromise” in Syria.
The Kremlin leader denied that Russian air strikes were hitting moderate opposition groups rather than Islamic State militants.
Syrian forces are said to have made significant advances against rebels.
Government gains in Idlib, Hama and Latakia provinces were on October 11 reported both by Damascus and opposition activists.
The main battlefront is currently close to the key highway that links the capital with other major cities, including Aleppo, and Bashar al-Assad’s forces are believed to be seeking to cut off rebels in Idlib.
In the interview with Rossiya-1 broadcast on October 11, Vladimir Putin said Russia’s aim was to “stabilize” the government in Damascus.
He stressed that without Moscow’s support for Bashar al-Assad there was a danger that “terrorist groups” could overrun Syria.
Bashar al-Assad’s government was currently “under siege”, Vladimir Putin said, adding that militants were “at the edge of Damascus”.
The Russian president also urged other nations to “unite efforts against this evil [terrorism]”.
The US-led coalition – which has been carrying out its own air strikes in Syria – earlier said it would not be co-operating with Russia.
Russia, which began its strikes in Syria on September 30, said on October 11 its aircraft carried out more than 60 missions in the past 24 hours, and that ISIS was its main target.
Gunmen have abducted six of the International Committee of the Red Cross (ICRC) workers and one Red Crescent volunteer in north-west Syria.
The aid agency has had no contact with the gunmen, an ICRC spokesman says.
Earlier, Syrian state media said gunmen had opened fire on Red Cross staff travelling on the road between Sirmin and Saraqeb in Idlib province.
The ICRC says it has been struggling to gain access across Syria to provide aid to injured and displaced people.
“I am able to confirm that six ICRC staff members and one Syrian Arab Red Crescent volunteer have been abducted near Idlib in north-western Syria,” ICRC spokesman Ewan Watson told Reuters.
Gunmen have abducted six of the ICRC workers and one Red Crescent volunteer in north-west Syria
“We are calling for their immediate and unconditional release of this team which was delivering humanitarian assistance to those most in need – and we do that on both sides of the frontlines,” he said.
Ewan Watson declined to reveal the identity, gender or nationality of the abducted workers but they are believed to include both local and international staff, who are mainly medical specialists.
Syrian state news agency Sana earlier quoted an unnamed official as saying the workers were abducted and taken to an unknown location after gunmen blocked their path and shot at their convoy.
Ewan Watson was unable to confirm whether or not shots had been fired, but he said the team’s vehicles were also missing.
An ICRC statement said the vehicles they were travelling in had been clearly marked with the Red Cross emblem, “which is not a religious symbol”.
Another ICRC spokesman, Simon Schorno, told Associated Press the attack took place at around 11:30 on Sunday as the team was returning to Damascus.
Simon Schorno said the team had been in the field since October 10 to assess the medical situation and deliver aid in what he described as “a difficult area to go in”.
It is not yet clear who carried out the kidnapping, but Syrian state TV blamed it on what it called “armed terrorists” – a term it frequently uses to describe anti-government rebels.
On Saturday, the Syrian government began the evacuation of around 1,500 civilians, mainly women and children, from a rebel-held Damascus suburb besieged by the army for months.
Many of those coming out of Muadhamiya, south-west of Damascus, were said to be exhausted and traumatized.