Obama: Syria's Assad, Russia, Iran have blood on hands

"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.

"This blood and these atrocities are on their hands," he said, before admitting to reporters that he also asks himself whether the United States has done enough to halt the war.

"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? So that's a starting point."

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.

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

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 broker 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 regime suspended that operation, trapping thousands of civilians and rebel fighters in the city and increasing fears of a bloodbath to come.

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.