Bashar al-Assad, Russia and Iran have blood on hands: Barack Obama

President Barack Obama has declared that Bashar al-Assad's Syrian regime, Iran and Russia are responsible for the carnage in Aleppo and argued there was nothing Washington could have done to stop the war, short of a military takeover of Syria.

And he warned Assad, who has been engaged in a brutal civil war against opposition forces since 2011, that he will not be able to "slaughter his way to legitimacy."

"The world as we speak is united in horror at the savage assault by the Syrian regime and its Russian and Iranian allies on the city of Aleppo," he told an end-of-year news conference on Friday.

"This blood and these atrocities are on their hands," he said.

Obama also admitted to reporters that he had asked himself whether the United States did enough to halt the conflict.

"There are places around the world where horrible things are happening and - because of my office, because I'm president of the United States — I feel responsible," he said.

"Is there something I could do that would save lives and make a difference and spare some child who doesn't deserve to suffer?"

But the president said there had been no public appetite for the kind of large-scale US military intervention earlier in the war that he believes would have been the only way to halt it.

"Unless we were all in and going to take over Syria we were going to have problems," he said. "It sounded like the right thing to do but it was going to be impossible to do this on the cheap."

The US leader, who leaves office on January 20 to make way for President-elect Donald Trump, called for impartial observers to deploy to monitor efforts to evacuate civilians from the city.

Obama's White House has been engaged in a diplomatic effort to convince Russia to bring Assad to the table to negotiate a peace deal with the Syrian opposition.

But all attempts to secure a ceasefire have rapidly broken down, and now Russia is working with Turkey to oversee an evacuation of the last rebel-held pocket of Aleppo.

On Friday, the Syrian government suspended that operation, trapping thousands of civilians and rebel fighters in the city and increasing fears of a bloodbath to come.

UN Secretary General Ban Ki-moon warned that Aleppo had become "a synonym for hell" and urged "the parties to take all necessary measures to allow safe resumption of this evacuation process."

Senior US officials, including Secretary of State John Kerry and Washington's ambassador to the UN Samantha Power, have warned Assad against carrying out a Srebrenica-style massacre.

They have also warned the defeat of Aleppo will not end the civil war but instead foment extremism among Assad's opponents and have called for war crimes investigations.