US-backed fighters on Thursday gave the Islamic State group 48 hours to leave the battleground Syrian town of Manbij, after US-led air strikes nearby killed scores reported to be civilians.

Elsewhere, a monitor said at least 51 civilians were killed in bombing raids on rebel-held areas across Syria.

The 48-hour ultimatum was issued by the Arab-Kurdish Syrian Democratic Forces (SDF), which is fighting IS with coalition air support.

The force has been waging a major campaign since June to oust IS from Manbij, in Aleppo province, with US-led air support.

The deadline follows a major backlash after reports that at least 56 civilians, including children, were killed in air strikes by the coalition near Manbij on Tuesday.

"In order to protect civilian lives and property and to protect the town from destruction we announce that we accept the initiative under which besieged IS members would leave with their individual light weapons," said the Manbij Military Council, part of the SDF.

"This initiative is the last remaining chance for besieged members of Daesh (IS) to leave the town."

An SDF commander told AFP that the initiative was first floated last week by tribal leaders in Manbij, which is a key IS bastion.

"But we took this decision now after IS used residents as human shields, after the media pressure on us, and to protect whatever civilians are left in the town," he said on condition of anonymity.

The statement also urged civilians to try to leave Manbij or distance themselves from areas where clashes are taking place.

- Outrage over civilian deaths -

The Syrian Observatory for Human Rights, a Britain-based monitor, said civilians killed Tuesday were villagers fleeing fighting in Al-Tukhar, 14 kilometres (nine miles) from Manbij.

The anti-IS coalition has said it is investigating the allegations, and US Defence Secretary Ashton Carter on Wednesday pledged a "transparent" investigation.

Syrian activists called for international protests over the incident, and local demonstrations have already been held inside Syria.

The opposition Syrian National Coalition on Wednesday urged the US-led anti-IS alliance to halt its strikes to allow a thorough investigation into what it termed a "massacre".

Coalition president Anas al-Abdah said the alliance was responsible for the "crimes" in Manbij, which he said killed at least 125 civilians.

And there has also been international consternation, with the UN children's agency UNICEF saying it had received reports up to 20 children might have been killed in the incident.

"No matter where they are in Syria or under whose control they live -- absolutely nothing justifies attacks on children," said UNICEF's Syria representative, Hanaa Singer.

Rights group Amnesty International also expressed alarm and demanded "a prompt, independent and transparent investigation."

The Pentagon has acknowledged 41 civilian deaths in its strikes in both Syria and Iraq since 2014, but the Observatory has reported nearly 600 civilians killed in US-led raids in Syria alone.

- Further civilian casualties -

On Thursday afternoon, the Observatory reported quiet in Manbij, though it was unclear if the calm was temporary or in response to the SDF's ultimatum.

Earlier in the day, the monitor had reported additional US-led air strikes and it said the SDF had advanced inside the town overnight.

Elsewhere in the country, the Observatory said at least 51 civilians had been killed in bombardment of rebel-held areas.

It said at least 13 people, including three children, were killed in government air strikes and shelling on the Eastern Ghouta area outside the capital Damascus.

Another 23 people were killed in strikes in Idlib province, though it was not clear if they were carried out by the regime or its Russian ally.

Government bombardment also hit two neighbourhoods of the rebel-held east of Aleppo city, where 15 people were killed, among them six children, the Observatory said.

Opposition-held neighbourhoods of Aleppo have been effectively under siege for the past two weeks, after government forces severed the only remaining supply route into the east of the city.

The UN on Thursday called for a weekly 48-hour truce in Aleppo to allow aid deliveries to the besieged east.

Jan Egeland, the head of the UN-backed humanitarian taskforce for Syria, warned that more than 200,000 people in eastern Aleppo were "on the brink of starvation".

"Humanitarian convoys are ready, humanitarian workers are ready. We have the supplies. We need a break in the fighting," he said.