Washington: Outgoing US President Barack Obama on Friday harshly accused the Syrian regime, along with Moscow and Tehran, of slaughtering civilians in Aleppo.
"We have seen a deliberate strategy of surrounding, besieging and starving innocent civilians," CNN quoted Obama as saying during a year-end news conference.
"Responsibility for this brutality lies in one place alone: the Assad regime and its allies Russia and Iran," Obama said. "The blood for these atrocities are on their hands."
He added, "The Assad regime cannot slaughter its way to legitimacy."
Asked about the fact the US didn't get involved, Obama said, "Because I am President of the United States, I feel responsible."
But Obama said that after every option was examined "in painful detail", factors including the involvement of Russia and Iran and the lack of cohesion among the Syrian opposition required a full-scale military intervention in order to aid the situation.
"Unless we were all-in and willing to take over Syria, we were going to have problems," Obama said.
"I understand the impulse to want to do something, but ultimately what I've had to do is to think about what we can sustain, what is realistic. And my first priority has to be what's the right thing to do for America," he said.
"Based on hours of meetings ... where we went through every option in painful detail ... short of putting large numbers of US troops on the ground, uninvited, without any international law mandate, without sufficient support from Congress" it would have been impossible.
"We couldn't do it on the cheap," he said.
Obama's condemnation comes as an agreement allowing for the evacuation of the thousands of civilians left in the besieged city appeared to fall apart on Friday.
Forces loyal to the Assad regime have shelled eastern Aleppo and after years of fighting, his troops have managed to break through rebel defensive lines, seizing some of the last few Syrian rebel holdouts in recent days.
The operation to evacuate thousands of refugees was halted, according to the International Committee of the Red Cross. Various reports cited violations of the cessation of hostilities as reason for the interruption.