Beirut (AFP) - Al-Qaeda's Syrian affiliate led a fierce assault on an intelligence base in Aleppo city Monday night after regime attacks on civilian infrastructure in the city forced rebels to shut schools.
Along with other Islamist militants, the Al-Nusra Front set off a large tunnel explosion near the air force intelligence headquarters in the west of the northern city.
Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights, said the "huge blast" was followed by "intense clashes" as the rebels tried to storm the base.
He said government forces had been able to prevent the rebels from entering the headquarters, but that the fighting was ongoing.
A Syrian security official confirmed to AFP that there had been an explosion in a tunnel near the base, but said that regime forces had "blocked the assault, and that all was quiet now".
A witness living in regime-controlled areas of Aleppo city said he heard a "huge boom" followed by sounds of gunfire.
There were no immediate reports of casualties.
Abdel Rahman said the militants fired mortar rounds at government forces after the blast, and the loyalists retaliated while regime aircraft pounded rebel positions.
Just over a month ago, dozens of people were killed when Al-Nusra and other rebel groups used similar tactics to attack the same intelligence headquarters, first detonating a tunnel explosion and then attacking regime forces.
Setting off explosives at the end of tunnels dug into government-controlled areas has become a favoured tactic of Syria's rebels, especially in Aleppo.
- Schools and markets shut -
Earlier on Monday, the city's rebel education authority called on schools and teaching centres in opposition-controlled areas to suspend classes until the end of the week, according to a statement distributed by the Observatory.
It said the decision was made "out of concern for the safety of students and teachers" after an air strike on a school in the city's east on Sunday killed five children, three female teachers and a man.
Bloodstains and debris marked the entrances of the bombed-out school, and broken desks and glass were strewn across classroom floors.
An opposition activist at the Aleppo Media Centre told AFP that 135 schools and markets in rebel-held parts of the city were closed "until further notice".
"People are more afraid than usual and there are dozens of families who have fled to refugee camps in Turkey or are internally displaced in Aleppo," the activist said.
Regime aircraft also struck a rebel warehouse storing baking flour and humanitarian assistance in east Aleppo, the Observatory said.
The strike destroyed several hundred tons of flour that were to have been distributed among bakeries in rebel-held parts of the city.
Pictures published by the Observatory showed flour spilling out of shredded bags, mixing with sand and debris from the air strike.
Other regime air attacks on the city killed seven people on Monday, including a father and his two children.
Abdel Rahman told AFP that the Syrian regime's aerial campaign against rebel-held areas of Aleppo had intensified significantly.
"For two months, the army didn't make a single real breakthrough on the battlefield," so it compensated with more air raids, Abdel Rahman said.
Once Syria's economic powerhouse, Aleppo has been divided into government control in the west and rebel control in the east since fighting began there in mid-2012.
Both rebels and regime have been accused by rights groups of indiscriminate bombing.
More than 215,000 people have been killed nationwide since the beginning of Syria's conflict in 2011.