Russia, Iran and Turkey set up a three-way mechanism to ensure compliance of all sides.

The cut-off is a major challenge to the government's effort throughout the nearly 6-year-old civil war to keep the capital as insulated as possible.

Syrian TV showed live footage of a long convoy of ambulances and green buses driving out of Aleppo.

Activists and fighters trapped in the opposition's last sliver of territory in Aleppo said pro-government forces had struck their district with dozens of rockets since mid-morning.

"What Russia is sponsoring and doing is not counter-terrorism, it's barbarism," said U.S. Ambassador Samantha Power.

Monitoring groups and state media reported clashes up until the final minutes, and the most powerful rebel groups having yet to commit to the truce.

The image of the stunned and weary-looking boy, covered in dust and with blood on his face, encapsulates the horrors inflicted on the war-ravaged northern city.

The reinforcements will shore up Syrian President Bashar Assad's forces as fighting over control of the city intensifies.

British ambassador to the U.N. said the aid was "too little, too late."

Salih Muslim, the co-president of the PYD, said the only way forward was a decentralized Syria—any formula for that would be acceptable to the Kurds.