Women and children civilians with their luggage crowd a street in the eastern part of Aleppo on Dec. 15. (Ghith Sy/European Pressphoto Agency)

The dramatic evacuation of Syrian civilians and rebels from besieged eastern Aleppo was thrown into chaos Friday as Russia signaled that the convoys were ending even as thousands of people waited to be ferried to safer ground.

Opposition fighters remaining in the embattled city, meanwhile, remanned positions in preparation for a possible renewal of battles amid reports that Syrian-allied militiamen seized and possibly killed some civilians trying to flee before the window to escape closed.

The situation was further clouded when Turkey, which brokered the evacuation effort with Russia, insisted that the convoys from Aleppo have not ended despite Moscow’s announcement.

“There are a lot of people who want to leave,” Turkish Foreign Minister Mevlut Cavusoglu told reporters in Ankara.

In Washington, President Obama, citing international organizations, said “tens of thousands” of civilians are “still trapped” in Aleppo, despite Russian and Syrian claims to the contrary. At a White House news conference, he warned that the regime of President Bashar ­al-Assad “cannot slaughter its way to legitimacy.”

(Louisa Loveluck, Jason Aldag/The Washington Post)

The signs of showdown and panic quickly replaced the brief respite that began Thursday when the first buses and ambulances finally rolled out of Aleppo under a deal that ended the siege and allowed passage for thousands of people from the devastated city.

The arrangement effectively handed victory to Syrian government forces and delivered a huge blow to opposition groups in the nearly five-year conflict, leaving questions about whether they can regroup.

It now appears that Syrian troops and their allies — including Russia and Iranian-backed militias — are looking to cement their control over strategic Aleppo with a push through the last enclaves still nominally in rebel hands. Yet thousands of civilians remain in the crosshairs, spurring urgent appeals from U.N. envoys and Turkey to allow the evacuations to continue.

“Negotiations are ongoing” to keep the convoys moving, said Elizabeth Hoff, the Syria representative for the World Health Organization, one of the groups helping coordinate the evacuation. “There are a large number of women and children, and there are others, who are still inside and want to get out,” she told the Associated Press from Aleppo.

Lina Shamy, an activist holed up in eastern Aleppo, posted a Twitter message saying that “thousands of civilians [and] a lot of injured” were still in the districts.

“Aleppo is now a synonym for hell,” U.N. Secretary General Ban Ki-moon said. He urged the parties to resume the evacuation of civilians.

But Russia’s Defense Ministry signaled that the pullout was over and suggested that military operations were resuming.

“All women and children in areas controlled by rebel fighters have been taken out,” the Russian statement said. It quoted “evacuees on the last convoy” as saying that “all those who wished to had left the eastern neighborhoods of Aleppo.”

The statement said more than 9,500 people have been evacuated since Thursday, including 4,500 fighters and 337 wounded. Turkey placed the number of civilians at 7,500. Those figures could not be independently verified.

“In certain districts there remain groups of radical fighters and irreconcilable gangs, who are firing on Syrian forces,” the Russian statement said. “Units of the Syrian army have resumed” operations in the areas.

Meanwhile, the spokesman for the rebel faction Ahrar al-Sham claimed that Shiite militiamen allied with the Syrian government have detained civilians trying to flee Aleppo. The move could be an attempt to force rebels to allow safe passage for people in nearby Shiite villages who support Assad.

“Sectarian militias disrupted the evacuation of civilians in Aleppo, civilians mostly women & children were taken hostages by these militias,” the spokesman, Ahmed Kara Ali, tweeted.

Other rebels and opposition activists said government forces prevented buses from leaving eastern Aleppo and killed several people and detained many others. The claims could not be independently confirmed.

Yaser Kor, a local council member in Aleppo, said he left in a private vehicle as part of a convoy Friday morning that included civilians and rebels.

“It was horrible. It took us more than eight hours to leave. We got to the gathering point at midnight last night, and we only made it out after eight hours after holdups by the regime. It was terrifying,” he said from the ­rebel-controlled province of Idlib.

For those who managed to leave Aleppo, it was the culmination of a nonstop blitz that reduced rebel-held districts to a stark tableau of collapsed buildings and roads cratered by shelling.

Earlier Friday, the bus evacuations were halted when gunfire hit a convoy. There were completing claims about which side opened fire.

Violence also hit Syria’s capital. Syrian state TV reported that a young girl detonated a suicide blast inside a police station in Damascus. The report did not mention casualties.

In Aleppo, pro-Assad forces pushed rebel fighters into a sliver of territory during a relentless month-long offensive. Although the city’s evacuation will not halt the fighting in Syria, it marks a huge blow — tactically and symbolically — to rebel groups staring down the barrel of defeat.

With leverage now in Assad’s favor, his ally, Russian President Vladi­mir Putin, proposed holding new talks in Kazakhstan between rebel groups and the Syrian and Turkish governments.

Putin added that the Syrian military offensive in Aleppo was “unconditionally successful” and said the recapture of the city would overshadow the “symbolic” loss of the ancient city of Palmyra to the Islamic State last week.

Friday’s remarks were Putin’s first public reckoning with a surprise Islamic State offensive that retook Palmyra on Sunday.

It was a gut punch for the Kremlin and Assad, who had presented the city’s capture in March as an important victory for civilization.

Roth reported from Moscow and Murphy from Washington. Zakaria Zakaria in Istanbul and William Branigin in Washington contributed to this report.