At least 28 civilians have reportedly been killed and several wounded in US-led air strikes on the suburbs of the northern Syrian city of Manbij, according to a monitoring group.

The reports on Friday come a day after the US-led coalition announced it had enough evidence of civilian casualties from its attacks on the same area last week to launch a formal investigation.

Thursday night's air strikes targeted the town of al-Ghandour, controlled by the Islamic State of Iraq and Levant (ISIL) group according to the UK-based Syrian Observatory for Human Rights (SOHR), which documents daily human rights abuses in Syria.

The SOHR said the civilian death toll included seven children. Thirteen more people died in the same attack, but their identity remained unclear, the group added.

Ghandour lies 23km northwest of Manbij in the Aleppo governorate, a strategic waypoint between Turkey and the city of Raqqa, the de facto capital for ISIL, also known as ISIS.

US Central Command (CENTCOM), which oversees US military operations in the Middle East, said late on Thursday it had "initiated an assessment following internal operational reporting that a strike today near Manbij, Syria may have resulted in civilian casualties," confirming that there had been coalition air strikes there in the past 24 hours.

Last week, a separate coalition attack targeting the Tokhar area of Manbij killed at least 56 Syrian civilians, according to SOHR and local activists, in one of the highest death tolls from coalition air strikes yet.

READ MORE: Syria war- Dozens killed in 'US-led strikes' on Manbij

After examining "internal and external information" following the strikes, the coalition determined that there was sufficient credible evidence of civilian deaths to open a formal inquiry, spokesman Colonel Chris Garver said on Wednesday.

"The US coalition knows that it is very important for it to be seen as trying to respond to these allegations," Al Jazeera's Rosiland Jordan, reporting from Washington, DC, said.

"This is the third such investigation in the past 11 days."

'Death corridors'

The reports of further civilian deaths from coalition air strikes emerged amid setbacks to attempts by the US and Russia to cooperate militarily in the fight against ISIL.

Syrian forces loyal to President Bashar al-Assad, supported by Russia, made a strategic advance in recent days by shutting the last remaining supply route into the rebel-held eastern part of Aleppo.

Russia, an ally of the Syrian government, also announced a joint plan on Thursday to open up three passages to give rebels willing to surrender, and the more than 300,000 civilians in Aleppo, a way out.

Responding to the plan, however, the US said on Friday that Syria's peace process could be derailed if Russia's motives surrounding humanitarian corridors were not genuine.

US Secretary of State John Kerry said the announcement "has the risk, if it is ruse, of completely breaking apart the level of cooperation", but also added it could "open up some possibilities" if an agreement on the way forward was reached in US-Russia talks about Syria in the Swiss city of Geneva on Friday.

According to SOHR, only a dozen of Aleppo residents trickled out through one of the passages on Friday, while others wanting to flee were turned back by opposition fighters.

A correspondent for the AFP news agency in east Aleppo said streets were empty with residents holed up indoors and shops shuttered.

Ahmad Ramadan, from the opposition Syrian National Coalition, accused Russia and the Syrian regime of forcing civilians to flee through continued bombing raids.

"Aleppo residents are calling the corridors that Russia is talking about 'death corridors'," he told AFP.

The SOHR also said that Syrian government air strikes killed at least eight civilians in two eastern areas of Aleppo, warning the death toll could rise as more were trapped under the rubble.

UN: Leave Aleppo corridors to us

Also responding to Russia's plan, UN Special Envoy to Syria Staffan De Mistura urged Moscow to let the UN take charge of any humanitarian corridors created in the divided city.

"Our suggestion is to Russia to actually leave the corridors being established at their initiative to us," de Mistura told reporters, at a press conference in Geneva on Friday.

"The UN and humanitarian partners know what to do."

De Mistura also echoed calls by UN humanitarian chief Stephen O'Brien for a 48-hour truce to allow life-saving supplies into the city's rebel-held east, which has been surrounded by pro-government forces since July 17.

"How can you expect people to want to walk through a corridor, thousands of them, while there is shelling, bombing fighting," the UN envoy said.

Reporting from Gaziantep, on the Turkish side of the Syria-Turkey border, Al Jazeera's Mohammed Jamjoom said de Mistura's comments "echoed what we've heard from opposition activists up until this point.

"There are far more concerns at this hour than there are any type of guarantees."