BEIRUT (Reuters) - Al Qaeda’s powerful Syrian branch, the Nusra Front, announced on Thursday it was ending its relationship with the global jihadist network founded by Osama bin Laden, to remove a pretext used by world powers to attack Syrians.

The announcement came as Russia and President Bashar al-Assad’s government declared a “humanitarian operation” in the besieged rebel-held sector of Aleppo, opening “safe corridors” so people can flee Syria’s most important opposition stronghold.

Washington said that appeared to be an attempt to depopulate the city and make fighters surrender. The opposition called it a euphemism for forced displacement.

In the first known video statement ever to show his face, the leader of the Nusra Front, Mohamad al-Golani, announced that the group would re-form under a new name, with “no ties with any foreign party”.

The move was being made “to remove the excuse used by the international community -- spearheaded by America and Russia -- to bombard and displace Muslims in the Levant: that they are targeting the Nusra Front which is associated with al Qaeda,” he said. The group would now be called Jabhat Fatah al-Sham.

Golani appeared in the video flanked by two other Nusra Front figures, in front of a new white flag for the group. Nusra Front’s old flag was black, the color used by ultra-hardline jihadist groups such as al Qaeda and Islamic State.

Earlier on Thursday, bin Laden’s successor as Al Qaeda leader, Ayman al-Zawahri, gave the Nusra Front his blessing to break away. In his message, Golani thanked Zawahri for putting the interests of Syrians ahead of organizational concerns.

The move appeared to be an attempt to appeal to Syrians who have long had deep misgivings about Nusra’s links with al Qaeda and the presence of foreign jihadists in its ranks. It could alter the strategic alignment on the ground if the renamed Nusra gains acceptance among other rebel groups.

STRONG POSITION

But Assad and his Russian allies are unlikely to accept the rebranding as a reason to halt military operations that have put the Syrian leader in the strongest position on the battlefield for years.

The Nusra Front, one of the most powerful rebel forces in Syria’s five-year, multi-sided civil war, was excluded along with Islamic State from a U.S.- and Russian-backed ceasefire this year.

Nusra is listed as a terrorist organization by the United States and the United Nations. Assad’s other opponents have long said its presence gave the government and its Russian allies a pretext to abandon the truce and launch advances under the cover of anti-terrorist operations permitted under the ceasefire.

The U.S. State Department said Nusra Front fighters remained a legitimate target for U.S. warplanes for now.

“We’re gonna have to wait and see,” State Department spokesman John Kirby said. “We judge a group by what they do, not by what they call themselves.”

Western countries are worried that the announcement of safe corridors for people to flee Aleppo could herald a Russian-backed government assault on the city.

“This would appear to be a demand for the surrender of opposition groups and the evacuation of Syrian civilians from Aleppo,” Kirby said. “The innocent people of Aleppo should be able to stay in their homes safely, and to receive the humanitarian access, which Russia and the regime ... in principle have agreed.”

Syria’s largest city before the war, Aleppo has for years been divided into rebel and government zones. Asserting full control would be the biggest victory for Assad so far, and a potential turning point in a civil war that has killed hundreds of thousands, spawned the world’s worst refugee crisis and drawn in most regional and world powers.

Any assault on Aleppo would also probably wreck a diplomatic effort by Secretary of State John Kerry to negotiate military cooperation between the United States and Russia.

LEAFLETS

The Cold War-era superpowers are running separate military missions in Syria against their common foe Islamic State, but are on opposite sides in the wider civil war, with Moscow supporting Assad, and Washington saying he must step down.

Leaflets have been air-dropped on rebel-held parts of Aleppo since Wednesday, telling civilians they would be given safe passage out and providing maps to exit routes.

Slideshow ( 10 images )

Around 250,000 civilians are believed to be trapped in the rebel zone. The United Nations says food supplies will run out within weeks.

Assad said rebels who surrendered within three months would be amnestied. State television quoted the governor of Aleppo as saying three humanitarian corridors would be established for residents to leave. Russia said a fourth corridor would be set up in the north for surrendering rebels.

The British-based Syrian Observatory for Human Rights said helicopters had been dropping baby diapers and meal packs with Russian-language labels over rebel areas.

But Syria’s main opposition High Negotiations Committee wrote to U.N. Secretary General Ban Ki-moon denouncing the corridors as “a euphemism for Russia’s efforts to alter Aleppo’s demographics and ensure forced displacement”, which it called a war crime.

Several international relief agencies said exit corridors were not a substitute for aid access.

Slideshow ( 10 images )

“Some 250 to 400,000 civilians remain in what was once Syria’s largest city – not all of them want or are able to leave,” Mercy Corps said.

“If it is a genuine humanitarian proposal, then clearly it will be accompanied by an end to the bombing campaign,” the British ambassador to the United Nations, Matthew Rycroft, told reporters in New York. “Clearly, the U.N. and the rest of us cannot be complicit in anything else, for instance any form of emptying of Aleppo or preparing for an onslaught of Aleppo or indeed any continuation of this medieval siege of Aleppo ...”

CORRIDORS NOT OPEN

The proposed corridors did not appear to be open so far. Two rebels and aid workers contacted in besieged Aleppo said the army had fired at civilians in one of the safe corridors, in the Salah al-Din district. A doctor for a medical charity that operates in Aleppo also said the army had fired artillery at families gathering near another corridor, in the opposition-held Bustan al-Qasr neighborhood.

Hael Asi Hilal, head of the Syrian Red Crescent in rebel-held areas, said no family had been able to leave via any corridor due to snipers firing at them.

The army, backed by allied militia forces and air support from Syrian and Russian jets, meanwhile took more ground on the northern edge of the city. State television said the army had advanced in the Bani Zeid district, and the Observatory said pro-government forces were in full control.

The United States and Russia jointly sponsored the ceasefire earlier this year that led to U.N.-brokered peace talks. But that collapsed in May and since then government forces have been advancing with Russian support. Kerry’s talks with the Russians, aimed at building a system to jointly identify targets, have been largely fruitless.

A Russian deputy foreign minister, Sergei Ryabkov, said Washington’s stance would allow militants to regroup since it required a ceasefire before distinguishing between terrorists and other opposition groups.

“There is an element here of a political ruse at least,” he said.

One U.S. national security official said it was difficult to agree as long as Moscow’s and Washington’s wider objectives diverged.

“The Russians want to destroy ISIS (Islamic State) to save Assad,” the official said. “We want to destroy ISIS to eliminate a terrorist threat and start a political process to remove Assad, who President Obama has said must go.”