The U.S. State Department has expressed concerns about what Russia is calling a "large-scale humanitarian operation" in Syria, saying Moscow's announcement appears to be a demand for the surrender of militant groups and the forced evacuation of civilians from the besieged city of Aleppo.
At the United Nations, U.S. Ambassador Samantha Power said in a tweet that she was given a copy of a "chilling letter" that was being distributed to civilians who are trapped in opposition-held parts of eastern Aleppo.
Powers said the letter "warns Syrians to leave eastern Aleppo and entrust their lives to a government that has bombed and starved them."
Meanwhile, the United Nation's humanitarian-aid chief, Stephen O'Brien, warned on July 28 that the so-called "humanitarian corridors" proposed by Russia to help Syrians leave Aleppo must be used voluntarily and that their protection must be guaranteed.
The International Committee of the Red Cross said the plan must ensure that civilians who leave voluntarily can access shelter and humanitarian aid, "and that families are not split up."
It said civilians that decide to stay in the opposition-controlled parts of eastern Aleppo must also be protected, and that all sides in the conflict must allow humanitarian-aid agencies access to those civilians in order to assess their needs.
The Red Cross also said a "humanitarian pause" was needed in Aleppo as soon as possible to reach the most vulnerable civilians there -- including the sick, elderly, and wounded, as well as detainees.
Rights watchdog groups warned the Syrian government on July 28 that opening safe passages to civilians trapped in Aleppo did not give Syrian and Russian forces carte blanche to further blockade the opposition-controlled territory or target it with indiscriminate fire.
Nadim Houry, the deputy Middle East director at Human Rights Watch, said the safe-passage offer, "doesn't mean that the people who stay behind are legitimate military targets."
Human Rights Watch also noted that Russia and Syria were "bound by international humanitarian law, or the laws of war, which prohibits indiscriminate attacks."
Russian Defense Minister Sergei Shoigu said on July 28 that Russian and Syrian government forces were opening "humanitarian" corridors for civilians to leave Aleppo and offering passage for rebel fighters who lay down their weapons.
Shoigu mentioned three corridors as well as what he said were food and first-aid points outside the city.
The announcement came as Syrian government forces tightened their control around rebel-held areas of Aleppo, where some 250,000 civilians are thought to be trapped.
Many residents dismissed the offer, saying it was a choice between slow death if they stay behind or possible detention and death by torture if they try to leave.
In Damascus, President Bashar al-Assad on July 28 offered what he said was an amnesty to rebels who lay down their weapons and surrender to government forces during the next three months.