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One activist said government forces, determined not to let the attackers seize the helicopters and warplanes parked on the tarmac, were seeking to destroy the aircraft themselves by shelling them.

Fighting raged for hours inside the base, the Taftanaz military airport in Idlib Province, as rebels, including fighters from the jihadist groups Jabhet al-Nusra and Ahrar al-Sham, held parts of the facility, according to the videos and reports from the Syrian Observatory for Human Rights, an antigovernment group based in Britain with a network of contacts inside Syria.

The videos showed what appeared to be rebels in a commandeered armored vehicle driving near a fence on the base and firing at buildings in the distance, from which smoke could be seen rising, as well as fires raging near helicopters parked on the tarmac.

The base is an important asset for the government, which has increasingly relied on helicopters to assault rebels and to resupply troops fighting to contain the nearly two-year-old uprising against the Syrian president, Bashar al-Assad. The rebels' inability to seize the base despite having it surrounded for months has highlighted the limits of their gains, so entering it would be an important achievement.

Abu Moyaed, the leader of a rebel battalion participating in the attacks, said in an interview from Turkey that the fighters had entered the airport and destroyed armored vehicles and aircraft, but then withdrew.

"It's very hard to stay there," he said, asserting that the government had used surface-to-surface rockets to attack their positions, as well as airstrikes reported by the Observatory.

The rebel leader said that even though his fighters seized ammunition from the base, "to gain control over the airport costs us three times as much ammunition as what we're going to get from inside."

The fighters are operating on a minimal budget, eating just one meal a day, he added.

Rebels have seized numerous air defense bases, like the Marj al-Sultan military airport in the Syrian capital, Damascus, and raided them for weapons, but have faced a harder time retaining them when government forces counterattack.

The Syrian Observatory said that more than 15 helicopters were damaged during the fighting or were already out of order, and that rebels had detained 13 government soldiers and 11 members of pro-government militias.

"The battle is still raging," said Tarek Abdel-Haq, an activist in Idlib reached on Skype earlier on Thursday. "All that's left is the main building of the airport, but the regime is shelling the town of Taftanaz to force the rebels to retreat."

Mr. Abdel-Haq also said "the regime is shelling its own military airport to destroy the warplanes on the runway to make sure the rebels and people can't use them. The rebels just gained control of the ammunition depot inside the airport."

A member of the revolutionary council in Damascus said that the prisoner exchange announced on Wednesday, in which more than 2,000 people detained by the Syrian government were to be released in exchange for 48 Iranians held by rebels, was not yet complete.

The member, who gave only a nickname for safety reasons, Abu Iyad, said that about 1,000 prisoners had been released by the Syrian government on Wednesday, while the remainder were expected to be freed on Thursday.

The prisoner exchange was hailed by Turkey and Iran as an achievement in cooperation between an ally of Syria's government, Iran, and a backer of the opposition, Turkey. It came on the eve of a top-level meeting scheduled for Thursday in Cairo between Iran's foreign minister and another strong backer of the opposition, Egypt's president, Mohamed Morsi.