Syria's government and rebels accused each other of launching a deadly chemical attack near the northern city of Aleppo on Tuesday in what would, if confirmed, be the first use of such weapons in the two-year-old conflict.
U.S. President Barack Obama, who has resisted overt military intervention in Syria, has warned Assad in the past that any use of chemical weapons would be a "red line." There has however been no suggestion of rebels possessing such arms.
Syria's information minister said rebels had fired a rocket carrying chemical agents that killed 16 people and wounded 86. State television said later the death toll had risen to 25.
The pro-opposition Syrian Observatory for Human Rights, which monitors the conflict using a network of contacts in Syria, put the number of dead at 26, including 16 soldiers.
The reported death toll is far below the mass slaughter inflicted on the Iraqi Kurdish city of Halabja where an estimated 5,000 people died in a chemical attack ordered by former Iraqi President Saddam Hussein 25 years ago.
There was no immediate confirmation from Western governments or international organizations of a chemical attack, but Russia, an ally of Damascus, accused rebels of carrying out such a strike.
"We are very seriously concerned by the fact that weapons of mass destruction are falling into the hands of the rebels, which further worsens the situation in Syria and elevates the confrontation in the country to a new level," the Russian Foreign Ministry said in a statement.
Britain said its calculations would change if a chemical attack had taken place. "The UK is clear that the use or proliferation of chemical weapons would demand a serious response from the international community and force us to revisit our approach so far," a Foreign Office spokeswoman said.
A Reuters photographer said victims he had visited in Aleppo hospitals were suffering breathing problems and that people had said they could smell chlorine after the attack.
"I saw mostly women and children," said the photographer, who cannot be named for his own safety. He quoted victims at the University of Aleppo hospital and the al-Rajaa hospital as saying people were dying in the streets and in their houses.
President Bashar al-Assad, battling an uprising against his rule, is widely believed to have a chemical arsenal.
Syrian officials have neither confirmed nor denied this, but have said that if it existed it would be used to defend against foreign aggression, not against Syrians. There have been no previous reports of chemical weapons in the hands of insurgents.
Information Minister Omran al-Zoabi said rebels fired "a rocket containing poison gases" at the town of Khan al-Assal, southwest of Aleppo, from the city's southeastern district of Nairab, part of which is rebel-held.
"The substance in the rocket causes unconsciousness, then convulsions, then death," the minister said.
But a senior rebel commander, Qassim Saadeddine, who is also a spokesman for the Higher Military Council in Aleppo, denied this, blaming Assad's forces for the alleged chemical strike.
"We were hearing reports from early this morning about a regime attack on Khan al-Assal, and we believe they fired a Scud with chemical agents," he told Reuters by telephone from Aleppo.
Washington has expressed concern about chemical weapons falling into the hands of militant groups, either hardline Islamist rebels fighting to topple Assad or his regional allies.
Israel has threatened military action if such arms were sent to the Syrian- and Iranian-backed Lebanese Hezbollah group.
Zoabi said Turkey
and Qatar, which have supported rebels, bore "legal, moral and political responsibility" for the strike, a charge dismissed by a Turkish official as baseless.
In a televised news conference, Zoabi said Syria's military would never use internationally banned weapons.
"Syria's army leadership has stressed this before and we say it again, if we had chemical weapons we would never use them due to moral, humanitarian and political reasons," he said.
Syrian state TV aired footage of what it said were casualties of the attack arriving at one hospital in Aleppo.
Men, women and children were rushed inside on stretchers as doctors inserted medical drips into their arms and oxygen tubes into their mouths. None had visible wounds to their bodies, but some interviewed said they had trouble breathing.
An unidentified doctor interviewed on the channel said the attack was either "phosphorus or poison" but did not elaborate.
"The Free Syrian Army hit us with a rocket, we smelled something and then everyone got dizzy and fell down. People were falling to the ground," said a sobbing woman in a flowered veil, lying on a stretcher with a drip in her arm.
A young girl on a stretcher wept as she said: "My chest closed up. I couldn't talk. I couldn't breathe ... We saw people falling dead to the floor. My father fell, he fell and now we don't know where he is. God curse them, I hope they die."
A man in a green surgical mask, who said he had been helping to evacuate the casualties, said: "It was like a powder, and anyone who breathed it in fell to the ground."
Two weeks ago rebel fighters seized a police academy in Khan al-Assal, about five miles southwest of Aleppo, which was being used as an artillery base by Assad's troops.
But the Syrian Observatory said Assad's forces had since retaken at least part of the town.
A rebel fighter in Khan al-Assal said he had seen pink smoke rising after a powerful blast shook the area.
Ahmed al-Ahmed, from the Ansar brigade in a rebel-controlled military base near Khan al-Assal, told Reuters that a missile had hit the town at around 8 a.m.
"We were about two kilometers from the blast. It was incredibly loud and so powerful that everything in the room started falling over. When I finally got up to look at the explosion, I saw smoke with a pinkish-purple color rising up.
"I didn't smell anything, but I did not leave the building I was in," said Ahmed, speaking via Skype.
"The missile, maybe a Scud, hit a regime area, praise God, and I'm sure that it was an accident. My brigade certainly does not have that [chemical] capability and we've been talking to many units in the area, they all deny it."
Ahmed said the explosion was quickly followed by an air strike. A fighter jet circled a police school held by the rebels on the outskirts of Khan al-Assal and bombed the area, he said.
His account could not be independently verified.
Ahmet Uzumcu, head of the Organisation for the Prohibition of Chemical Weapons, said in Vienna he had no independent information about any use of such arms in Syria.
The World Health Organization, a U.N. agency, and the International Committee of the Red Cross, also said they had not been able to confirm the reports.