Turkish men from pro-Islamic groups lie on the ground next to a painted doll representing dead Syrian babies as they protest against the war in Aleppo, Syria, in Istanbul, Thursday, Dec. 8, 2016. Aleppo shakes with explosions and gunfire day and night in both the government and rebel sides. (AP Photo/Emrah Gurel)
Turkish men from pro-Islamic groups lie on the ground next to a painted doll representing dead Syrian babies as they protest against the war in Aleppo, Syria, in Istanbul, Thursday, Dec. 8, 2016. Aleppo shakes with explosions and gunfire day and night in both the government and rebel sides. (AP Photo/Emrah Gurel) Credit: ap — Emrah Gurel

Beirut — Hundreds of men have disappeared after fleeing rebel-held districts in the Syrian city of Aleppo, the United Nations said on Friday, amid allegations that armed groups on both sides have abducted and even killed civilians who tried to leave.

Tens of thousands of people have flooded out of eastern Aleppo since forces loyal to President Bashar al-Assad began a final push to retake the city.

Rebel fighters appeared to have reinforced the front line of their shrunken enclave on Friday, putting up their strongest fight in weeks. But an official with the armed opposition, speaking on the condition of anonymity because he was not authorized to speak publicly, said their numbers were badly depleted.

In Paris, Secretary of State John Kerry said U.S. and Russian teams would meet today in Geneva in an effort to “save the city of Aleppo” and discuss a plan to safely evacuate civilians and rebel fighters.

“We are close; we’re not there yet,” Kerry said of urgent talks this week with Russian Foreign Minister Sergei Lavrov. “Tomorrow, I will have the team from America, at President Obama’s direction, to be in Geneva with the Russians and we will, I hope, come to some kind of arrangement where we can see how civilians may be protected and what can happen with the armed opposition.”

The Geneva meeting, with diplomatic and military officials from both sides, will restart U.S.-Russia talks on Syria that Washington suspended in October in protest over Russian bombing of civilians and infrastructure.

Today, Kerry plans to meet in Paris with counterparts from Europe and the Middle East who are supporting the rebels along with the United States.

Thousands of rebels and as many as 250,000 civilians had held out in eastern Aleppo for four years through bombardment and siege. But many have fled since Syrian and Iran-backed pro-Assad militia fighters swept through three-quarters of the rebel enclave in an offensive that began on Nov. 15.

The U.N. human rights spokesman, Rupert Coville, said Friday in Geneva that his office was hearing “worrying allegations” that hundreds of men had disappeared in the exodus.