Fighters captured the Taftanaz base after months of laying siege as part of rebel strategy to choke off regime's airpower

Rebels fighting the regime of Bashar al-Assad scored a significant victory on Friday when they took control of one of Syria's most important northern airbases, seizing tanks, helicopters and large amounts of ammunition.

Fighters had laid siege to the Taftanaz base near the town of Idlib for months. After seizing several buildings on Wednesday they stormed the sprawling complex on Friday morning. "As of now, the rebels are in full control of the airbase," Idlib-based activist Mohammad Kanaan said.

A video from the scene shows jubilant rebels ripping down a large poster of Assad at the entrance gate. Others wave from the upper story of a barracks. Trucks carry off boxes of ammunition. The bodies of four government soldiers lay sprawled in a muddy pit.

In another video captured Sunni government soldiers claim their Alawite officers fled the base early on Friday, abandoning them. Government forces appear to have removed most of the 60 helicopters stationed at Taftanaz – leaving around 20 that were apparently non-functional.

In recent months the rebels have systematically targeted airbases across the country in an attempt to choke off the government's key military advantage: air power. Taftanaz has been used to launch repeated helicopter strikes against opposition strongholds in nearby Aleppo, Syria's divided northern city, and elsewhere.

Fighters from Jabhat al-Nusra and other radical Islamist groups spearheaded the Taftanaz attack, punching through when previous attempts had failed. The US claims Jabhat al-Nusra is allied to al-Qaida. The organisation does not deny its al-Qaida links, but is trying to eschew its bloody past in Iraq by engaging in community outreach programmes and avoiding sectarian rhetoric.

The development will alarm western countries, who are increasingly concerned after almost two years of fighting at the rise of Islamist militias in Syria.

Ultimately the seizure may do little to halt airstrikes by government jets, many of which come from bases further south. But it will undoubtedly embolden the rebels, who are still besieging other bases, and whose command structure is improving. The regime responded by launching punitive air strikes on Taftanaz, which lies near the highway between Aleppo and the capital Damascus.

Rami Abdul-Rahman, director of the UK-based Syrian Observatory for Human Rights, said this was the first major military airbase to fall into rebel hands. Kanaan, the Idlib activist, said the rebels seized helicopters, but added that most if not all were already damaged from the fighting. "The regime bombed them to keep the rebels from using them," he said.

Yezid Sayigh, senior associate at the Carnegie Middle East Centre in Beirut, said Taftanaz's capture would help the rebels as they try to secure a continuous area in the north. But he played down the broader military significance, pointing out it had taken the rebels many months to take the base. "This is a tactical gain rather than a strategic gain," Sayigh said.

Meanwhile Lakhdar Brahimi, the UN-Arab league envoy to Syria, met Russian and US diplomats on Friday in an attempt to find seeking a political solution to the conflict. There was little prospect of a breakthrough. The Syrian government recently accused Brahimi of "flagrant bias" after he suggested that a peace deal would probably only be possible if Assad agreed to step down from power.

As part of the meeting Russia's deputy foreign minister Mikhail Bogdanov held talks with the US deputy secretary of state William Burns, Washington's top diplomat. Russia has been Syria's most influential ally since the uprising began, providing Assad with diplomatic and military support. But in recent months the Kremlin has concluded that Assad, sooner or later, is finished.

The UN refugee agency said on Friday that it is concerned about the severe winter conditions faced by some 612,000 Syrian refugees in Jordan, Lebanon, Turkey, Iraq and Egypt. There has been no let up in the flow of thousands of people a day across the borders, it said, as the region experiences snow and shivering temperatures. At least one million Syrians have been internally displaced, with many in dismal conditions.

"Many of those arriving have been barefoot, with their clothing soaked, and covered in mud and snow," agency spokesman Adrian Edwards said in Geneva, referring to new refugee arrivals in Jordan.