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 a northern town 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 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.