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.
At the United Nations, Britain, France, Portugal, Germany and the United States said they would begin drafting a Security Council sanctions resolution on Syria. "We believe that the time has come for the Council to take further action," Britain's Deputy UN Ambassador Philip Parham told reporters.
US President Barack 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-Zor and surrounded a mosque in Latakia on Thursday.
"The future of Syria must be determined by its people, but President Bashar al-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."
UN 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.
A UN report issued in Geneva 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.
The Syrian Revolution Coordinating Union, an activists' group, said security forces fired machineguns near a mosque in Latakia which was surrounded by armored vehicles.
In New York, Syria's UN Ambassador Bashar Ja'afari accused the United States and European powers of waging a "diplomatic and humanitarian war" against Syria by imposing sanctions and demanding that Assad quit.
Britain's Parham told reporters the proposed UN sanctions would include an arms embargo, asset freezes and travel bans. But veto-holders Russia and China have so far opposed concrete measures against Damascus.
Nadim Shehadi of London's Chatham House think-tank said the shift in tone from Washington and Europe was significant, since it may encourage Syrians who saw previous calls for Assad to reform as an indication of support for him.
"The previous messages from the West to Bashar al-Assad were ambiguous," Shehadi said. "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."
Rosemary Hollis, a Middle East politics lecturer at London's City University, said the move could "rattle the regime, they will feel very isolated".
It may take time, however, for the diplomatic broadside, backed by sanctions, to have an impact on the 45-year-old president, who took power when his father President Hafez al-Assad died 11 years ago after three decades in office.
He has so far brushed off international pressure and survived years of US and European isolation following the 2005 assassination of Lebanese statesman Rafik al-Hariri, a killing many Western nations held Damascus responsible for.
Despite the escalating international rhetoric and Western sanctions, no country is proposing to take the kind of military action NATO forces launched in Libya to support rebels fighting Muammar Gaddafi. That action has helped them take much of the country.
However, Syria's economy, already hit by a collapse in tourism revenue, could be further damaged by Obama's announcement. US sanctions will make it very difficult for banks to finance transactions involving Syrian oil exports.
It will also make it challenging for companies with a large US presence, such as Shell, to continue producing crude in Syria -- although the impact on global oil markets from a potential shutdown of Syria's 380,000 barrels per day oil industry would be relatively small compared to Libya.
Assad says the protests are a foreign conspiracy to divide Syria and pledged last week his army would "not relent in pursuing terrorist groups".
EurActiv with Reuters