WASHINGTON: The United States will declare the Al-Nusra Front, a group battling to overthrow Syria's President Bashar al-Assad, a “foreign terrorist organisation,” documents showed Tuesday.

The State Department has not formally announced the move to blacklist the group, but posted the declaration in the Federal Register, in a document that described the Al-Nusra Front as an alias of Al Qaeda in Iraq.

The Al-Nusra Front is one of the most effective of several armed groups fighting to overthrow the Syrian regime, raising concerns in the West that hardline group is hijacking the 21-month-old revolt.

The group has claimed responsibility for recent suicide bombings that killed scores of people, and has said it hopes to replace the Assad family's four-decade-old dictatorship with a strict Islamic state.

The State Department linked the group to Al Qaeda in Iraq, which has carried out scores of massive bombings targeting Shia civilians and regularly targeted US forces before their withdrawal a year ago.

“We've had concerns that Al-Nusra is little more than a front for Al Qaeda in Iraq, (which) has moved some of its operations into Syria,” State Department spokeswoman Victoria Nuland said earlier on Monday.

She said the State Department would “have more to say about this” in the coming days.

Declaring the Al-Nusra Front a terrorist group would entail freezing its assets and banning Americans from any transactions with the group, which could complicate efforts to provide aid to Syria's fragmented rebellion.

Earlier on Monday, militants led by the Al-Nusra Front seized a strategic army base in the northern Syrian province of Aleppo.

An AFP correspondent reporting from the scene said many of the fighters hailed from other Arab countries and Central Asia.

The move to declare the Al-Nusra Front a terror group comes ahead of the so-called Friends of Syria meeting set to take place in Morocco on Wednesday.

US Secretary of State Hillary Clinton had planned to attend the meeting but cancelled her trip on Monday due to illness. Deputy Secretary of State William Burns was to attend in her place.

Arab and Western states backing the revolt were to discuss the political transition in the event of Assad's fall, and mobilising vital humanitarian aid as winter sets in.

Since the last meeting (in July) in Paris, the number of people killed has risen from 16,000 to more than 42,000, according to the Observatory.

US has repeatedly called on Assad to step aside but says it has not provided arms to the rebels for fear they could end up in the hands of radical groups.