"Warplanes are bombing Taftanaz military airport in an attempt to destroy it," a statement said.
Capturing Taftanaz is an important gain for the rebels who control vast swathes of Syria's north and east and are battling President Bashar al-Assad's forces in most major cities and on the outskirts of Damascus.
"This is the largest airbase to be seized since the revolt began" nearly 22 months ago, Abdel Rahman told AFP by phone.
The rebels had previously taken the relatively small Hamdan airport in Albu Kamal on the border with Iraq in the east, and Marj al-Sultan military airport in Damascus province.
The Taftanaz assault was led by jihadist fighters from the Al-Nusra Front, Ahrar al-Sham and Islamic Vanguard battalions, as well as other rebel groups, the Observatory said.
It said eight soldiers and two regime militiamen were killed, as were seven rebels. Many soldiers and officers fled the base at dawn.
Government forces managed to evacuate most of the 60 helicopters deployed there, leaving behind 20 that are no longer serviceable, the Observatory said.
The news came as UN-Arab League special envoy Lakhdar Brahimi met Russian Deputy Foreign Minister Mikhail Bogdanov and US Undersecretary of State William Burns in Geneva.
After more than five hours of talks, Brahimi expressed an urgent need to end the conflict, but reported no major progress.
"We all stressed the need for a speedy end to the bloodshed and the destruction and all forms of violence in Syria," he told reporters.
"If you are asking me whether a solution is around the corner, I'm not sure if that is the case," was all he would say when asked if specific progress had been made.
Brahimi is due to brief the Security Council later this month.
Friday's discussions came after Syria accused Brahimi of "flagrant bias," casting doubt on whether he could remain international mediator.
Damascus lashed out at the veteran Algerian diplomat for describing as "one sided" proposals Assad made on Sunday for a "political solution."
In comments to the BBC, Brahimi attacked Assad's plan to keep fighting rebel "terrorists" and ignore opposition groups tied to them.
He also questioned the decades-long rule by Assad's family, remarking that "what people are saying is that one family ruling for 40 years is a little bit too long."
Pro-government Al-Watan newspaper denounced him as a Western "pawn," and the foreign ministry accused him of "flagrant bias for those parties known to be conspiring against Syria and its people."
There had been hope that Friday's talks could produce a clearer idea of how to move towards a transitional government in Syria, where the UN estimates more than 60,000 people have died since the March 2011 outbreak of the revolt.
The Observatory put Friday's provisional death toll at 62 -- 22 rebels, 21 soldiers and 19 civilians.
The Austrian, Danish, Irish and Slovenian EU foreign ministers urged the UN in a joint letter on Friday to refer the conflict to the International Criminal Court, saying "horrendous crimes" have taken place with "no consequences for the perpetrators."
Despite wintry conditions, thousands of Syrians staged nationwide protests denouncing "death camps," referring to refugees suffering in tent settlements in neighbouring states, which this week were battered by storms.
The UN said 612,134 Syrians had been registered as refugees in the region or were in the process of being recorded, a sharp rise from the 509,550 announced on December 11.
"They left with their wives and children fearing kidnappings, bombings and snipers. They dreamed of a tent safely away from the gangs of Assad, but instead were met with a slow death," activists wrote on the Facebook page Syrian Revolution 2011.