While Assad's opposition blame his troops for the deadly university bombing, the government says extremist group al Nusra is responsible.
Establishing the facts - who bombed what and how many people died - is almost impossible in a country where everyone mistrusts journalists.
Former Syrian prime minister Riyad Hijab says that the Assad regime is an "enemy of God" in his first public appearance since his defection.
A US official has said that there is no evidence Syrian rebels or Assad's regime used chemical weapons.
Russia has accused Syrian rebels of using chemical weapons in an attack today, and saying it was an extremely alarming and dangerous development.
"A case of the use of chemical weapons by the armed opposition was recorded early in the morning of March 19 in Aleppo province," the Russian Foreign Ministry said after President Bashar al-Assad's government and rebels accused each other of launching a deadly chemical attack.
"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 ministry said in a statement.
– Foreign Office spokeswoman
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.
Eyewitnesses in Aleppo have told ITV News that earlier today they experienced a strong smell, followed by white smoke which has caused choking among those in the city, following reports of a chemical attack.
The Free Syrian Army's rebel command has said that Bashar Al-Assad's forces carried out a chemical weapon attack near Aleppo earlier today using a long-range missile.
The Syrian Observatory for Human Rights has reported that 26 have been killed in the attack in Aleppo, including 16 soldiers.
"Sixteen Syrian regular army soldiers were killed in Khan al-Assal," Rami Abdelrahman, head of the Syrian Observatory for Human Rights, told Reuters. "Ten more died in hospital but I cannot confirm if they are civilians or soldiers."
Syria's Information Minister Omran al-Zoabi has said that the chemical weapon fired is "first act" by interim government.
A Syrian rebel commander has denied firing a chemical weapon in Aleppo, and has accused state forces for launching a scud missile with chemical agents.
"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.
Then suddenly we learned that the regime was turning these reports against us," said Qassim Saadeddine, a senior rebel and spokesman for the Higher Military Council in Aleppo.
"The rebels were not behind this attack" he said.
A Syrian minister has said that Turkey and Qatar bear responsibility for what Syria is calling a chemical attack by rebels.
The attack reportedly killed 16 and wounded 86.
The Syrian information minister has said that the rebels in Aleppo have fired a chemical weapon, calling it a "dangerous escalation".