Syrian rebels announced a new evacuation deal for Aleppo civilians and the injured, after shelling and air strikes sent terrified residents running through the city's streets.

However a source close to Syria's government denied the development, and the Syrian Observatory for Human Rights said hostilities were ongoing after an earlier agreement collapsed.

Under the initial plan, thousands of civilians and rebel fighters were due early Wednesday to evacuate the east of Syria's second city, scene of some of the worst violence in more than five years of war across the country.

But cold and hungry civilians who had gathered before dawn to leave were instead plunged back into a familiar nightmare.

"Bombing is ongoing, no one can move. Everyone is hiding and terrified," activist Mohammad al-Khatib told AFP from inside east Aleppo.

"The wounded and dead are lying in the street. No one dares to try and retrieve the bodies."

Officials from the Nureddin al-Zinki and the hardline Ahrar al-Sham rebel groups confirmed to AFP that a new truce deal had come into effect after hours of violence Wednesday.

"A ceasefire has come into effect in Aleppo after negotiations between the Russians and the Turkish Red Crescent," said Yasser al-Youssef, a Nureddin al-Zinki political official.

"The first group of civilians and wounded people will leave at dawn on Thursday," Youssef told AFP.

He said an agreement on rebels had also been reached but news of the deal was denied by a source close to Syria's government.

"There is no agreement, the negotiations are ongoing," the source said.

As air strikes and artillery fire shook the city earlier Wednesday, AFP's correspondent in rebel areas of Aleppo saw panicked civilians running for cover.

The Britain-based Observatory reported "very intense clashes on every front line" and said at least two people had been killed in rebel areas.

State television said rebel rocket fire on government-controlled areas had killed at least seven people.

Former Al-Qaeda affiliate Fateh al-Sham Front said Wednesday that one of its suicide bombers detonated a car bomb at a regime position in southern Aleppo.

A source with knowledge of efforts to resuscitate the ceasefire deal earlier said Syria, opposition backer Turkey, and regime allies Russia and Iran were locked in talks.

Moscow has blamed rebels for violating the ceasefire, saying it now expected opposition resistance in Aleppo to end in the next "two to three days".

A source close to the government said Damascus objected to the number of people leaving, claiming rebels had sought to raise it from 2,000 to 10,000.

The agreement came amid international concern about the plight of civilians in the city, and as the UN said it had reports of atrocities being committed by advancing government forces.

In Istanbul, more than a thousand people protested outside the Iranian consulate to denounce the actions of regime allies Tehran and Moscow and demand the evacuation of civilians from Aleppo.

In Kuwait around 2,000 demonstrated in front of the Russian embassy while in Paris several hundred people gathered in support of the victims of Aleppo.

The lights on the Eiffel Tower were also switched off in what Paris Mayor Anne Hidalgo was a protest at the "unbearable" situation for civilians in Aleppo.

The UN said Tuesday it had credible reports of at least 82 civilians, including 11 women and 13 children, being executed in recent days.

And the UN's Commission of Inquiry for Syria said it had received reports opposition fighters were blocking civilians from fleeing Aleppo and using them as human shields.

It was unclear how many civilians remained in rebel territory, after an estimated 130,000 fled to other parts of Aleppo during the government advance since mid-November.

Syria's conflict has evolved from anti-Assad protests into a multi-front war that has killed more than 312,000 and drawn in world powers on all sides of the war.