The army said Thursday it has retaken full control of Syria's devastated second city Aleppo, scoring its biggest victory against opposition forces since the civil war erupted in 2011.

The announcement came after a landmark evacuation deal that put an end to a ferocious month-long offensive waged on east Aleppo by government forces and allied militia.

Earlier, the Red Cross said more than 4,000 fighters had left rebel-held areas of the city in the final stages of an evacuation.

The loss of east Aleppo is the biggest blow to Syria's rebel movement in the nearly six-year conflict, which has killed more than 310,000 people.

It puts the government in control of the country's five main cities: Aleppo, Homs, Hama, Damascus, and Latakia.

President Bashar al-Assad's victory in Aleppo is a boon for his allies in Moscow and Tehran and a defeat for the opposition's backers, including Turkey, Saudi Arabia, Qatar, and some Western states.

Because of the intensity of these global rivalries -- particularly between Russia and the United States -- the international community struggled for years to respond to the bloodshed in Syria.

"The liberation of Aleppo is not only a victory for Syria but also for those who really contribute to the fight against terrorism, notably Russia and Iran," state news agency SANA quoted Assad as saying before the army announcement on Thursday.

The evacuation effort had been hampered by heavy snowfall and freezing temperatures, leaving evacuees waiting in unheated buses for hours.

"Overnight between Wednesday and Thursday, in one of the last stages of the evacuation, more than 4,000 fighters were evacuated in private cars, vans, and pick-ups from eastern Aleppo," said Ingy Sedky, the spokeswoman in Syria for the International Committee of the Red Cross.

She said about 34,000 people had left rebel areas of Aleppo under the evacuation plan.

The United Nations said it had deployed observers to monitor the final evacuations, under a Security Council resolution adopted on Monday.

Jens Laerke, spokesman for the UN humanitarian agency, said 31 staff had been assigned for monitoring at the crossing point at Ramussa, the government-held district of southern Aleppo through which evacuation convoys have been leaving.

"It's been a very difficult night. The weather is really harsh, and people are leaving in hundreds of private vehicles at different levels of disrepair," he told AFP.

Heavy snowfall from Wednesday, which blanketed Aleppo and the surrounding countryside, had slowed down the evacuations.

"The bad weather, including heavy snow and wind, and the poor state of vehicles... mean things are moving much more slowly than expected," Sedky said.

Rebel forces, who seized control of east Aleppo in 2012, agreed to withdraw from the bastion after a month-long army offensive that drove them from more than 90 percent of their former territory.

The deal was brokered by Russia, which launched air strikes in support of Assad's regime last year, and Turkey, which has supported some rebel groups.

As part of the Aleppo evacuation deal, it was agreed some residents would be allowed to leave Fuaa and Kafraya, two Shiite-majority villages in northwestern Syria that are under siege by the Sunni Muslim rebels.

About 1,000 people have been able to leave the villages in recent days.

The evacuation of Aleppo's rebel sector is a pivotal moment in a war that has triggered a major humanitarian and refugee crisis.

As well as a major strategic gain for Assad, the rebel withdrawal from Aleppo has given fresh impetus to international efforts to end the conflict.

Russia, Iran and Turkey agreed this week to guarantee Syria peace talks and backed expanding a ceasefire, laying down their claim as the main powerbrokers in the war.

Repeated attempts at peace have failed, but UN envoy Staffan de Mistura has said he hopes to convene fresh talks in Geneva in February.

Formerly the beating heart of Syria's commercial and cultural industries, Aleppo has been split since July 2012 between rebels in the east and the government in the west.

East Aleppo became a powerful symbol for Syria's opposition, which set up its own administration to run schools, electricity, and water there.

Opposition fighters lobbed rockets into government-held territory, and regime forces battered the east with air strikes and artillery.

Moscow's military intervention in support of Assad marked a major turning point in the war.

Defence Minister Sergei Shoigu said Thursday the Russian air force has killed 35,000 fighters in Syria since it began in September last year.

Turkey launched its own campaign in Syria in late August in support of pro-Ankara rebels, with the aim of ousting Islamic State (IS) militants as well as Kurdish militia from areas near its border.

Turkish air strikes killed at least 47 civilians including 14 children Thursday in the IS-held town of Al-Bab, which Turkish forces have been seeking to capture for weeks, the Syrian Observatory for Human Rights monitoring group said.

The raids came a day after 14 Turkish soldiers were killed by jihadists around Al-Bab, in the country's biggest loss of the campaign so far.

Prime Minister Binali Yildirim vowed Thursday to press on, saying: "Turkey is in the midst of a great struggle -- our fight against terror continues both in our country and outside our borders."

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