Evacuation suspended after group leaving east Aleppo was stopped and attacked before being sent back, witnesses say.

Four people who were part of a convoy evacuating the besieged districts of east Aleppo were killed by Syrian government forces, witnesses told Al Jazeera, before the agreement to evacuate tens of thousands of fighters and civilians was suspended.

Two men, who were part of the group, also said the entire convoy was briefly detained and told Al Jazeera’s Amro Halabi, reporting from east Aleppo, that security forces opened fire on evacuees before forcing them to return to the war-torn city.

“They forced us out of the vehicles, forced us to lay flat on the ground, stripped us of our clothes and then we heard gunfire,” one of the men said.

“When we looked up, we noticed three or four people were killed.”

We're around 1000 they took us after we reached regime's areas,handcuffs us,killed 4 & told us that it's pay back then we came back#Aleppo — Zouhir Al-Shimale (@ZouhirAlShimale) December 16, 2016

Following the incident, the Syrian government suspended the evacuation that began on Thursday as part of a ceasefire deal to move civilians to rebel-held Idlib province.

The government also accused rebels of trying to smuggle out prisoners and heavy weapons.

More than 40 buses and ambulances had evacuated almost 3,000 people from east Aleppo to neighbouring Idlib province on Thursday. At the time of the deal’s suspension, the government-run SANA news agency reported that more than 8,000 residents of eastern Aleppo, among them fighters, had been evacuated.

Tens of thousands of people were still trapped inside east Aleppo and were too scared to leave their besieged districts, Al Jazeera’s Halabi said.

“Now the people are afraid and they are running away from the meeting point where they were supposed to gather in order to take the buses out of the besieged east Aleppo districts, they are in a state of horror and shock,” he said.

READ MORE: Syria’s Civil War Explained

Zouhir al Shimale, an independent journalist in east Aleppo, was part of the convoy that was held up. He told Al Jazeera that in addition to being beaten, the civilians were also robbed of cash before being sent back.

“They took us after we reached regime areas, handcuffed us, killed four, and told us its payback. Then we came back,” he said on Twitter.

In another tweet, he said militias robbed evacuees “of all their money” before blocking them.

Al Jazeera’s Charles Stratford, reporting from the city of Antakya in neighbouring Turkey, said that he received reports that Iranian militiamen were the ones who blocked the convoy.

“They blocked this convoy of around 20 vehicles from leaving east Aleppo. Another witness said those militias opened fire, took some men off the buses and stripped search them.”

He also described the situation in east Aleppo as “desperate”, before adding “a lot of the residents were scared to come out of their houses, they are very scared about moving independently into government-controlled areas”.

Russia, which backs the Syrian government, denied the convoy was stopped.

The evacuation began a month to the day after Syrian government forces launched a major offensive to retake all of Aleppo, and will hand the Syrian President Bashar al-Assad his biggest victory in more than five years of civil war.

On Friday, US President Barack Obama warned Assad that he would not be able to “slaughter his way to legitimacy” and also put responsibility on the Syrian regime’s Iranian and Russian backers for the civilian deaths in Aleppo.

“The world, as we speak, is united in horror at the savage assault by the Syrian regime and its Russian and Iranian regime on the city of Aleppo,” said Obama. “This blood and these atrocities are on their hands.”

Outgoing UN chief Ban Ki-moon called Aleppo a “synonym for hell”, before adding that he told the Security Council “we have collectively failed the people of Syria”.

Aleppo, Syria’s second largest city and once a key cultural and economic hub, has been divided between government forces and rebels since 2012.

In a video message to Syrians, Assad said the “liberation” of Aleppo was “history in the making”.

But Pawel Krzysiek, communications chief of the International Committee of the Red Cross (ICRC), said there was “fear, desperation and anxiety” on the streets.

“People are waiting with children and elders,” said Krzysiek from Aleppo.

“It’s really cold here. They are on the streets burning plastic to keep warm. I can see hundreds, if not thousands, waiting to be evacuated.”