Syrian residents flee violence in the Maysaloun neighbourhood of the northern embattled Syrian city of Aleppo . (Youssef Karwashan/AFP/Getty Images)

Hundreds of men have disappeared after fleeing rebel-held districts in the Syrian city of Aleppo, the United Nations said 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 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 F. Kerry said U.S. and Russian teams would meet Saturday 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.

On Saturday, 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 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.

“Given the terrible record of arbitrary detention, torture and disappearances, we are of course deeply concerned,” he said.

The Syrian Observatory for ­Human Rights, a monitoring group, estimates that some 80,000 people have left rebel areas in recent weeks as government forces have advanced. Residents have said that men of fighting age were being separated from their groups by soldiers or militiamen and that some were being taken to a nearby airport for interrogation.

Coville also said Friday that armed opposition groups have blocked civilians from leaving — in some cases firing at them as they tried to flee.

“During the last two weeks, ­Fatah al-Sham Front and the Abu Amara Battalion are alleged to have abducted and killed an unknown number of civilians who requested the armed groups to leave their neighborhoods, to spare the lives of civilians,” he said. He was referring respectively to the al-Qaeda-linked group formerly known as Jabhat al-Nusra and to an Islamist militant group allied with it against the Assad forces.

Kerry also charged rebels with some of the civilian killings, saying that “even people who are supposedly on their side [are] shooting to prevent them from leaving so they stay there as human shields.”

On Thursday night, Lavrov said Syrian government military operations in Aleppo had stopped. On Friday, however, he said that it was only a temporary “pause” to allow some civilians to leave and that the offensive had begun again.

With the Geneva meeting, “there is a very good opportunity to agree on how to finalize the settlement in eastern Aleppo by the withdrawal of all militants from there,” Lavrov said at a news conference in Hamburg, where he is attending an international conference.

Lavrov, and Kremlin spokesman Dmitri Peskov in Moscow, accused the Obama administration of “strange” behavior in continuing to criticize Russia while Kerry is negotiating.

Lavrov denied that Russia was delaying an agreement on Syria until Obama leaves office. But, he said, President-elect Donald Trump’s statements on terrorism are “more clear” than those of Obama.

Officials preparing for Trump’s administration met in Washington this week with members of the Syrian Opposition Coalition, the political group that the Obama administration hopes can return to the negotiating table for talks with the Assad government over a transition government in Syria.

Coalition Secretary General Abdul Ilah Fahad said in a telephone interview that his group and the Trump team “agreed on the necessity of being a strong partner in countering terrorism and the necessity of countering the Iranian action in Syria, and to push for protecting civilians and [for] a political transition.”

The Syrian government said Friday that it was ready to resume dialogue with the opposition but without external intervention or preconditions, according to a statement carried by the state-run Syrian Arab News Agency.

Kerry also said he hoped that an Aleppo agreement would pave the way for a wider cease-fire, the safe flow of humanitarian aid to Syria, and a resumption of political talks between the government and the opposition that were aborted last spring when the latter protested ongoing Syrian and Russian bombing of civilian areas.

Since then, their loss of territory has weakened the rebels’ negotiating position, which had included the insistence that Assad leave power, while U.S. demands about Assad’s early departure appear to have weakened.

Fahad said that any proposal to retain Assad “will complicate things much more.” It will “increase Iran’s influence” in Syria, he said, and will never be accepted by countries in the region who are supporting the opposition.

In statements Friday, Russia also denounced what it described as the Obama administration’s “decision” to sent portable antiaircraft missiles to the rebels. Statements by Lavrov and Peskov apparently referred to the defense budget authorization passed this week by the House, which removed long-standing language explicitly prohibiting the provision of those weapons to “any entity” fighting in Syria.

While some lawmakers have pushed to send the weapons, the administration has long opposed it on grounds that the weapons might be transferred to terrorist groups.

DeYoung reported from Paris.