BEIRUT -- A Syrian rebel group that the United States has labeled an affiliate of al Qaida in Iraq appeared Thursday to be on the verge of overrunning a government air base that’s used to launch helicopter strikes against rebel-held areas in Syria’s north.

According to the London-based Syrian Observatory for Human Rights and video of the fighting posted on the Internet, the offensive at Taftanaz, an air base near the road that links the Syrian cities of Idlib and Aleppo, was being led by the Nusra Front, which the State Department designated a terrorist organization last month.

Ahrar al Sham, another rebel group that, like Nusra, has called for establishing an Islamic state in Syria, is participating in the offensive, according to its Internet postings.

State Department spokeswoman Victoria Nuland offered an upbeat assessment of the rebel advance Thursday, saying that the likely takeover of the base was a sign that the abilities of the "armed opposition" were growing. She added that the capture of the base would help the rebel side by cutting off the Assad regime’s ability to resupply its troops in the north.

She didn’t acknowledge in her remarks that the assault on the base was spearheaded by Nusra, whose Arabic name is Jabhat al Nusra, and it was unclear whether she knew of the group’s role. State Department spokesmen didn’t respond to later requests for comment.

Nusra and Ahrar al Sham have proliferated across Syria in recent months, leading rebel advances and eclipsing the role of more moderate rebel groups. In its designation of Nusra as a terrorist organization, the State Department said that the group, which announced its presence a year ago with massive bombings in Damascus, was just another name for al Qaida in Iraq, a radical Sunni Muslim group responsible for the deaths of hundreds of U.S. troops and thousands of Iraqi civilians.

The declaration complicated U.S. support for the rebels, who are organized in a variety of fighting groups, each of which reports to its own commanders with its own political and religious beliefs. Obama administration officials had hoped that the designation would help channel aid to the rebels away from Nusra to more moderate factions. The designation, however, was immediately denounced by Sheikh Mouaz al Khatib, the head of the newly founded Syrian National Coalition for Opposition and Revolutionary Forces, which the United States has said should lead any post-Assad government.

Since then, Nusra influence appears only to have grown, with the group assuming a crucial role in rebel offensives across the country, including the current campaign to erode the government’s air power by seizing air bases.

In addition to Taftanaz, Nusra is leading the fighting at al Nayrab military airport in Aleppo, the country’s largest city. Islamist fighters also are likely to be playing a key role in the siege of a military airfield in the eastern province of Deir el Zour, where Nusra fighters led assaults that a McClatchy reporter witnessed in November.

Jeff White, a military analyst who studies Syria for the Washington Institute for Near East Policy, said rebels had hit at least one aircraft that was attempting to land at the Deir el Zour military airport in recent days and that they were closing on Mengh air base, another of the government’s northern bases. Earlier this year, Ahrar al Sham led fighting that partially shut Abu Duhor, another air base in the northern part of the country.