Armed Syrian rebels claim to have entered Aleppo, the first time rebels have entered the country's second-largest city since government forces retook the city in 2016.
Rebel forces launched a surprise offensive this week, capturing a number of villages and reigniting a conflict that had been largely static for years.
"Our forces have begun to enter the city of Aleppo," said a statement from the newly formed rebel coalition, the Military Operations Command.
The Syrian army said it was facing a "major attack" launched by "terrorist organizations" armed with heavy weaponry and drones.
The rebel "Military Operations Command" said earlier they took control of the Syrian government's Military Scientific Research Center on the outskirts of Aleppo city after "intense clashes with regime forces and Iranian militias".
Earlier in the day, an artillery shell hit student housing at Aleppo University, killing four people, according to Syria's state news agency SANA, which blamed opposition factions for the attack. Rebel group spokesman Hassan Abdulghani dismissed the allegations by Syrian government media as "baseless lies".
An employee of Aleppo University, speaking anonymously for security reasons, confirmed that an artillery shell hit the second floor of a dormitory with students inside at the time. A video circulating on social media shows youths running from a dormitory on the Aleppo University campus and carrying an injured individual, CNN reports.
On Thursday, at least 15 civilians, including six children and two women, were killed and 36 others wounded in airstrikes and shelling in rebel-held areas of Aleppo and Idlib countryside, according to the White Helmets, a volunteer rescue group. Iranian state media said an Islamic Revolutionary Guard Corps (IRGC) brigadier general Kioumars Pourhashemi was also killed in the city.
In a phone call with his Syrian counterpart to discuss the escalation, Iranian Foreign Minister Abbas Araghchi accused the United States and Israel of "reactivating" the rebels and "stressed Iran's continued support" for the Syrian government and military.
Kremlin spokesman Dmitry Peskov called on the Syrian authorities to "quickly restore order in this area and restore constitutional order." Both Iran and Russia are key allies of Syria.