Renewed airstrikes killed at least 10 civilians Tuesday in Syria's besieged eastern part of Aleppo as they were fleeing government advances in the opposition held-enclave, opposition groups reported. The United Nations said up to 16,000 people have already been displaced in recent days of fighting.

France called for an urgent Security Council meeting to discuss the situation in Aleppo, where fighting continued as government forces seek to break down rebel defenses in the deeply divided city, once Syria's commercial center.

The opposition Britain-based Syrian Observatory for Human Rights said new airstrikes left at least 10 civilians killed, while a rescue group in eastern Aleppo put the figure at 25. The Syrian Civil Defense, a team of first responders, said the airstrikes hit a group of civilians fleeing to Bab al-Nairab district from government advances in the north.

In swift and dramatic advances, Syrian government and allied troops pushed their way into northern parts of opposition held eastern Aleppo in the last couple of days, touching off a wave of panic and flight from the besieged enclave. Many of the fleeing civilians headed to government and Kurdish-controlled areas while others were driven deeper into the remaining rebel-held zones.

The U.N. Humanitarian Chief Stephen O'Brien said he was "extremely concerned" about the fate of civilians, calling the situation of Aleppo "deeply alarming and chilling."

"There are no functioning hospitals left, and official food stocks are practically finished in eastern Aleppo," O'Brien said in a statement. "It is likely that thousands more will have no choice but to flee should fighting continue to spread and intensify over the coming days."

Residents of the besieged, eastern part of Aleppo reached by telephone reported thousands of civilians have moved to its southern district, cramming into alleyways and empty or deserted buildings. "They are not safer from airstrikes, but are at least seeking refuge from the government gangs," said Abdulkafi Alhamdo, a teacher who has remained in rebel-held Aleppo. Alhamdo said many of the fleeing civilians fear revenge attacks from the advancing government, amid reports of arrests from people living in government-held areas.

Alhamdo said 11 empty apartments in his building were already occupied by the newly displaced, many of them women.

"Those have escaped death miraculously," he told The Associated Press from Aleppo, where airstrikes and shelling continued. "They called it a death trip."

Amnesty International said it has received reports of Syrian security forces detaining men in Aleppo areas that have returned to government control. The London-based human rights group warned of the potential for revenge attacks, arbitrary detention, torture, harassment and kidnappings against people formerly living under opposition control. The reports could not be independently confirmed.

O'Brien, of the U.N., said civilians in western government-held Aleppo have also come under indiscriminate shelling by opposition groups — violence that has displaced about 20,000 people in recent weeks in that area.

"The parties to the conflict in Syria have shown time and again that they are willing to take any action to secure military advantage even if it means killing, maiming or starving civilians into submission in the process," O'Brien said.

The government has imposed a tight siege on the eastern districts of Aleppo since July, with no aid going in for the last four months while punishing aerial bombings have been hitting hospitals and infrastructure. A brief breach by the rebels lasted nearly two weeks amid fighting around the area. The U.N. estimates that 275,000 people are trapped in the rebel-held enclave, which has now been cut into half with the new government advances.

The Russian Ministry of Defense said Tuesday that "half" of eastern Aleppo has been "liberated," estimating that 80,000 people living there were "freed."

France's foreign minister called for an urgent U.N. Security Council meeting to try to stop the fighting in Aleppo and bring in humanitarian aid.

Jean-Marc Ayrault said he will meet Wednesday in Paris with the head of Aleppo's district councils, Brita Haj Hassan. "More than ever, it is urgent to put in place a stop to hostilities and allow unhindered access to humanitarian aid," the French foreign minister said.

France has supported Syrian opposition groups resisting Assad's Russian-backed forces.