People allowed to flee besieged areas of the Syrian city of Aleppo using new humanitarian corridors must be given protection, the Red Cross says.
Aid workers must also be allowed access to civilians and ensure that families who choose to leave are kept together, the agency added.
Russia, Syria’s ally, says three routes will be open for civilians and unarmed rebels and a fourth for armed rebels.
Meanwhile the US says it is assessing if a coalition strike killed civilians.
Unverified, graphic images from a village near Manbij, northern Syria, have been posted on social media.
A statement from the US military confirmed the coalition fighting so-called Islamic State had carried out strikes in the area and said it was investigating further.
Around 300,000 people are trapped in rebel-held eastern Aleppo.
Russian Defence Minister Sergei Shoigu described the corridors as a “large-scale humanitarian operation”.
He said the move was “first and foremost to ensure the safety of Aleppo residents”.
But the move has been treated cautiously by much of the international community.
US state department spokesman John Kirby said the exercise appeared to be an attempt to force the evacuation of civilians and the surrender of militant groups.
“What needs to happen is the innocent people of Aleppo should be able to stay in their homes, safely, and to receive the humanitarian access which Russia and the [Syrian] regime have agreed in principle,” he said.
The UN’s Under Secretary General for Humanitarian Affairs, Stephen O’Brien, also said he was not yet convinced that the move would be in the best interests of those in need.
“We want to make sure that, if as an offer and a contribution, certain routes are being offered, then they would have to be absolutely guaranteed by all the parties that they are safe, and that nobody will be forced to use those routes involuntarily or to go in a direction away from the city or to any other place which was not of their own choosing,” he said.
The International Committee of the Red Cross (ICRC) said it welcomed any respite for civilians but that departures must be voluntary and the safety of those who decided to stay must be guaranteed.
Food running out
Robert Mardini, ICRC director for the Near and Middle East, said what was needed was “a humanitarian pause” in all areas of Aleppo affected by violence.
“Our teams need to reach communities in eastern Aleppo now, especially families and the most vulnerable, such as the elderly, the sick and injured, and detainees,” he said.
The UN said on Monday that food supplies in Aleppo were expected to run out in mid-August and many medical facilities continued to be attacked.
Mr Shoigu said the three corridors for civilians and unarmed fighters would have medical posts and food handouts. He said he would welcome the co-operation of international aid organisations.
The fourth corridor, in the direction of Castello Road, would be for armed militants, although Mr Shoigu complained that the US had not supplied information about how the rebel Free Syrian Army units it supports had separated from jihadist al-Nusra fighters.
Reports on Thursday said that government forces had taken control of more areas of the city, in the Bani Zeid neighbourhood.
Syrian President Bashar al-Assad has also offered an amnesty for rebels laying down arms and surrendering within three months.
The offer came in a decree issued on Thursday, the state-run Sana news agency reported.
“Everyone carrying arms… and sought by justice… is excluded from full punishment if they hand themselves in and lay down their weapons,” it quoted the decree as saying.
There have been several presidential amnesty offers in recent years.