The army said on 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.

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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.

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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.

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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.