Another mortar misfired and could be heard spinning out of control and landing far short of the base.

Disappointed, Abu Marwan explained the rebels were trying to use homemade artillery rounds. But then he proudly displayed an enormous, 30-foot-long cannon his men had captured from the Syrian military.

"We will use their own weapons against them," he said, pointing to a plate engraved with Syrian military logos, fixed to the base of the cannon.

Sizing up the siege

Though the rebels appear to have trapped the government troops, they have also suffered losses.

At least two top commanders in the Northern Storm Brigade have been wounded. Their loss has propelled Abu Marwan, a surprisingly shy pilot in his 20s who defected from the Syrian military more than a year ago, into a top leadership position.

"I may be the youngest fighter in the group," the former lieutenant said with a smile, after declining to reveal his exact age.

Videos secretly filmed by opposition activists show Syrian soldiers almost casually walking around tanks and tents in the airbase.

Last Sunday, rebels said the defenders broke through the siege lines to receive a much-needed delivery of supplies under cover of darkness. First, warplanes roared overhead, they said, sending people running for cover. It was then that a helicopter flew in, picked up wounded soldiers, and dropped off supplies before leaving.

A special forces soldier who escaped from the base and surrendered to rebels last week told CNN the troops inside were segregated between mostly Sunni Muslim foot soldiers like himself and officers who were predominantly Alawite, the same religious minority as Syrian president Bashar al-Assad.

"They discriminate between the Alawites and Sunnis," said the soldier, a 22-year-old man from the southern city of Deraa, who asked not to be named.

"The Sunnis go to the front almost as if they are human shields ... and all the Alawites stay behind."

The soldier said there were large stores of food and even supplies of electricity for commanders.

As for weapons, "we have so many weapons at the airport I can't even count. But despite that I could (see) that (the rebels) are not afraid."

The siege forces appear to have received fresh reinforcements in past weeks.

Last weekend, a CNN team of journalists traveled towards the front lines escorted by a young activist from an opposition media center in the nearby anti-regime town of Azaz.

Shortly after passing a line of bombed out buses and trucks serving as a barricade, the reporters were intercepted by a small group of fighters, who covered their faces once they spotted cameras.

"I am very sorry that you traveled all this way, but you have to leave," their hooded leader explained politely, yet firmly, in fluent English. "This is a restricted military zone."

The guide from the Azaz Media Center later explained that the fighters were members of an Islamist rebel group affiliated with the Muslim Brotherhood. He said another group of fighters spotted around several parked pickup trucks were members of al-Nusra Front.

Last month the U.S. government blacklisted al-Nusra, declaring it a terrorist organization linked to al Qaeda. However, al-Nusra rebels continue racking up victories on the battlefield and appear to be attracting growing respect from many supporters of Syria's weary armed opposition.