Washington (AFP) - President Barack Obama declared Friday 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.

"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.

- 'Synonym for hell' -

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.

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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.

- 'Right approach' -

But Washington has not been able to mediate the start of a credible peace process, nor separate so-called "moderate rebels" from Al Qaeda-linked extremists.

"I cannot claim that we've been successful and so that's something that, as is true with a lot of issues and problems around the world I have to go to bed with every night," Obama said.

"But I continue to believe it was the right approach given what realistically we could get done."

More than 310,000 people have been killed since early 2011, when Assad brutally repressed anti-government protests and provoked a civil war.

More than half the population has been displaced, with millions becoming refugees, placing a huge burden on neighboring countries and sparking a political crisis in Europe.

Meanwhile, the Islamic State jihadist group has taken advantage of the chaos to seize part of eastern Syria and set up a "capital" in the city of Raqa.

US troops and warplanes are helping locally recruited militias fight IS in the east, but have not intervened against Assad, who has Russian military backing.