BEIRUT – Syrian rebels stepped up their siege of a government helicopter base and clashed with soldiers near Aleppo’s international airport on Friday, part of an effort to chip away at the air power that poses the biggest challenge to their advances against the regime of President Bashar Assad.

That airborne threat came into stark relief the same day, when a government airstrike on the town of al-Safira south of Aleppo killed 14 people — most of them women and children, activists said. More than 21 months into Syria’s conflict, the Assad regime is counting more than ever on its air force to block rebel gains.

Rebels in the north, a region largely clear of government troops, realize this and have launched campaigns to seize all the area’s airports, hoping such a move will protect their forces and the civilians who back them.

This push in many ways represents the mismatched nature of Syria’s civil war, with numerous but lightly armed rebels fighting a highly sophisticated army, albeit one badly weakened by defections.

Rebels say they have surrounded four airports in the northern province of Aleppo. They have posted dozens of videos online showing fighters shooting mortars, homemade rockets and sniper rifles at targets inside the bases.

It remains unclear whether rebels will be able to seize any of the bases soon, but they have managed to stop air traffic at one and limit movement at others by firing on all approaching aircraft with heavy machine guns.

“The airports are now considered the most important thing the rebels can focus on because all of the strikes now come from the air,” said Aleppo activist Mohammed Saeed via Skype.

Saeed said clashes between rebels and government soldiers raged until Friday morning around the Mannagh helicopter base near the Turkish border. He said other rebel groups continued to hold positions around the Kuwiras military airport southwest of the city of Aleppo and clashed with soldiers near Aleppo’s international airport and Nerab military airport.

Rebels have numerical superiority and support from most of the population in the far north, making it easy for them to surround and cut the ground supply lines to government bases.

But Assad’s forces still control the air, responding to rebel gains with airstrikes on their positions or residential areas, a tactic rebels consider collective punishment against civilians who back the revolt.

The rebels remain largely helpless against regime air power. But many groups now have heavy caliber anti-aircraft guns they say act as a deterrent to low-flying aircraft.

The fall of any of Aleppo’s airports would give a psychological boost to rebels and give them greater freedom of moment since ground forces often shell from inside the airports.

It would not stop the airstrikes, most of which are carried out by jets from the central province of Hama or near Damascus.