Obama: 'Assad cannot slaughter his way to legitimacy'

US President Barack Obama declared Friday that Bashar al-Assad's Syrian regime and its Iranian and Russian backers are solely responsible for the slaughter of civilians in Aleppo.

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

"This blood and these atrocities are on their hands."

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 civil war against rebel forces since 2011, that he will not be able to "slaughter his way to legitimacy."