It was a dramatic sharpening of international rhetoric — major states had urged Assad to reform rather than resign. But with no threat of Western military action like that against Libya's Muammar Qaddafi, the five-month-old conflict between Assad and his opponents seems likely to grind on in the streets.

Putting faith in sanctions rather than force, Obama ordered Syrian government assets in the United States frozen, banned US citizens from operating or investing in Syria and prohibited US imports of Syrian oil products.

Though UN Secretary-General Ban Ki-moon said Assad had assured him on Wednesday that military operations were over, activists said Syrian forces carried out further raids in Deir Al-Zour and surrounded a mosque in Latakia on Thursday.

"The future of Syria must be determined by its people, but President Bashar Assad is standing in their way," Obama said. "His calls for dialogue and reform have rung hollow while he is imprisoning, torturing and slaughtering his own people."

In a coordinated move, European Union foreign policy chief Catherine Ashton called on Assad to step aside and said the EU was preparing to broaden sanctions against Syria.

United Nations human rights investigators said Assad's forces had carried out systematic attacks on civilians, often opening fire at close range and without warning, and committing violations that may amount to crimes against humanity. Crimes against humanity are considered the most serious of all international human rights violations after genocide.

A new UN report recounted complaints of indiscriminate shooting and of wounded people being put to death with knives or by being dumped in the refrigerated rooms of hospital morgues.

Nadim Shehadi of London's Chatham House think-tank said the shift in tone from Washington and Europe was significant, since it may give heart to Syrians who saw previous calls for Assad to reform as an indication of support for him, albeit ambiguous. "Now the West has hit at the very basis of the idea of his power, by telling him that we don't believe in you any more and you should leave."