Satellite images, released by Amnesty International, show the destruction of Aleppo during months of violence.
ITV News has obtained exclusive footage of life in one of Syria's makeshift hospitals where volunteers risk their lives to treat the wounded
While Assad's opposition blame his troops for the deadly university bombing, the government says extremist group al Nusra is responsible.
Air strikes around the northern Syrian city of Aleppo have killed at least 40 people, most of them civilians, the Syrian Observatory of Human Rights told Reuters.
The pro-opposition monitoring group said there were at least six strikes on the outskirts of Aleppo and nearby towns. Dozens of people were wounded, it said.
"Some of the strikes in the neighbourhood of Tareeq al-Bab appeared to be targeting rebel headquarters but instead the rockets fell in a busy street and caused heavy civilian casualties," Rami Abdelrahman, head of the Observatory, said.
Dozens of Syrian rebels were killed and hundreds were injured in a military operation to regain control of the crucial city of Aleppo dubbed the "Storm of the North", Syrian official sources claimed today.
The battle came five days after the army and its Lebanese ally Hezbollah seized the city of Qusair in western Syria, and could be a sign of reversing momentum in the conflict.
“Almost a hundred terrorists and salafists were killed at the start of the operation," a top security officer said on condition his name would not be used.
“The Storm of the North operation will continue until all Aleppo is liberated from all the terrorists and killers. Nothing is going to stop us."
Aleppo, Syria’s largest metropolis, has been divided into rebel-held and loyalist-controlled sections for a year.
UN Secretary-General Ban Ki-moon has announced the United Nations will launch an investigation - as requested by the Syrian government- into allegations that chemical weapons were used in Syria.
"I have decided to conduct a United Nations investigation into the possible use of chemical weapons in Syria", Ban told reporters.
He said the investigation will look into "the specific incident brought to my attention by the Syrian government".
The US ambassador to Syria said there was no evidence of chemical weapons use in the conflict and added that the administration are examining the reports, according to the Associated Press.
– Robert Ford, the US ambassador to Syria
So far, we have no evidence which substantiates the reports that chemical weapons were used yesterday. But I want to underline that we are looking very carefully at these reports.
The World Health Organization (WHO) said that it would send medical supplies to the Syrian city of Aleppo tomorrow, but could not verify if chemical weapons or some other toxin had been used there.
"At this stage we cannot confirm the use of chemical weapons, nor what agent, if any, was used," WHO spokesman Tarik Jasarevic told Reuters in Geneva. "Tomorrow (Wednesday) morning WHO will send medical supplies (for trauma cases) to Aleppo from its prepositioned stocks in Tartous."
U.N. Secretary-General Ban Ki-moon and the director general of the independent Organization for the Prohibition of Chemical Weapons (OPCW) said they were deeply concerned about the alleged used of chemical weapons in Syria.
Ban's office said in a statement:
The Secretary-General remains convinced that the use of chemical weapons by any party under any circumstances would constitute an outrageous crime.
The OPCW is a Hague-based body charged with overseeing the implementation of the Chemical Weapons Convention.
Foreign Secretary William Hague has re-iterated that the government is looking into reports of chemical weapon use in Syria - and warned about potential repercussions.
UK is aware of reports alleging use of chemical weapons in #Syria, and is looking into them. Use would draw strong international response
Malcolm Chalmers, research director at Royal United Services Institute (RUSI), says the use of chemical weapons in Syria would be a “game changer”.
“If indeed there is confirmation of chemical weapon use, then we would move much closer to some sort of American military intervention,” he said.
He added: “Both the regime and the rebels have said that chemical weapons have been used…
“Both the [US] President and the Secretary of State Clinton have made it clear that such use would be unacceptable and it would lead to unspecified consequences – but it’s pretty clear those consequences would be military.”