US President Barack Obama has for the first time called on Syria's President Bashar al-Assad to step down over a brutal crackdown by Syrian forces on demonstrations.
The president has signed an executive order imposing sanctions to freeze all Syrian government assets under US jurisdiction and bar US transactions with the Assad government.
At the same time, the leaders of Britain, France, Germany and the European Union also issued statements calling on President Assad to leave.
EU diplomats were scheduled to discuss sanctions in Brussels on Friday, opening the way for any new measures to go into effect as early as next week.
In a written statement, Mr Obama said the Syrian leader's calls for dialogue and reform have rung hollow while he is imprisoning, torturing, and slaughtering his own people.
While the United States had tightened sanctions against members of Syria's government and had called on Mr Assad to embark on democratic reforms or to get out of the way, Mr Obama's statement marks the first unambiguous demand for him to go.
A Syrian government spokesman accused Western governments of increasing the tension in the country.
President Assad told UN Secretary-General Ban Ki-moon that military and police operations against pro-democracy protesters have stopped, but activists and residents reported more killing overnight and more shooting on Thursday.