REYHANLI, Turkey - Buoyed by the ouster from northern Syria of Islamist extremists and the arrival of new weapons, including U.S.-supplied TOW antitank missiles, Western-backed rebels have gone on the offensive in two battle zones and say they've gained ground against forces loyal to the government of President Bashar al-Assad.

The Assad government has responded by stepping up an aerial bombing campaign that has driven tens of thousands of civilians to flee.

In Aleppo, where government forces seemed on the verge of capturing the city a month ago, a rebel offensive that began in mid-April is moving ahead slowly on all fronts, activists claim. Western-backed and Islamist fighters now are besieging the city's most important security facility, the headquarters of air force intelligence, after capturing three buildings near the compound.

To avoid the regime's barrel bombs - homemade weapons that can't be aimed accurately - rebels are concentrating forces at the front lines, where Assad's forces rarely drop the devices, apparently for fear of hitting their own troops.

About 60 miles to the south, rebels are using the same tactic to keep the main north-south highway closed at a small town called Murak, where they claim to have destroyed a large number of tanks, many with the help of newly arrived antitank weapons.

The rebels first seized control of the road in early February, cutting the main supply route from the government's weapons depots in Hama to cities in the north, including Aleppo. The action also forced the government to abandon 18 checkpoints in Khan Sheikhoun, a town just to the north of Murak.

That ended a yearlong government siege of Khan Sheikhoun, whose usual 80,000 population had doubled to 160,000 with people who had fled combat elsewhere. It also allowed the rebels to turn the tables and besiege three bases that government forces had used to shell nearby villages, commanders and activists said. Now the government is having to resupply the bases by parachute. The supplies often land in rebel-held areas.

"The fighters are from the area, and they know how to move around. They can advance to a few hundred meters of tanks and destroy them," said Capt. Amin Danawar, a defected Syrian army officer who was a frontline commander during part of the offensive.

The government responded by opening a bombing campaign against Khan Sheikhoun that drove most of the civilians from the town. Khalid el Yousef, 42, a farmer and a brigade commander in the Western-back Free Syrian Army, said only about 5,000 people remain in the town, and that most of them spend much of their time underground, cowering in primitive shelters from the daily bombing and artillery shelling.

The offensives and other factors, including what rebels say is growing financial support, have bolstered rebel morale.