Syria's military has used fighter jets to fire on Aleppo, the country's second city, the UN Supervision Mission in Syria (UNSMIS) spokeswoman told AFP on Wednesday.

"Yesterday they (UN observers) saw firing from a fighter aircraft" on Aleppo, said Sausan Ghosheh. "They are jets."

The UN observers, deployed in Syria to monitor a UN-Arab League-brokered truce that never took hold, have witnessed heavy fighting in Aleppo, Ghosheh told AFP.

"We are deeply concerned about the heavy fighting in Aleppo. The last 72 hours saw a significant increase in the level of violence in the southeast neighbourhoods of Aleppo, around Salaheddin area."

Fighting between regime forces and rebels has raged in Aleppo, Syria's commercial hub, since July 20.