President Barack Obama today for the first time called on Syrian president Bashar al-Assad to step down after a crackdown by Syrian forces on demonstrations against the Assad family's 41-year reign.
The president also imposed a fresh round of US sanctions to freeze all Syrian government assets under US jurisdiction and to bar US individuals or companies from transactions with Dr Assad's government.
The US sanctions and demand for Dr Assad's exit were followed by a call from Britain, France and Germany - three of the European Union's leading powers - for the Syrian president to go.
The moves are intended to increase pressure on Dr Assad, who has used his military and security forces to attack protesters seeking an end to four decades of authoritarian rule by the Syrian president and his late father, Hafez al-Assad.
While the United States has previously called for Dr Assad to embark on democratic reforms or to get out of the way, Mr Obama's statement marks the first explicit call for him to go.
His decision to make a direct appeal for Dr Assad's departure appears to reflect a US conclusion he is incapable of reform as well as growing condemnation of the Syrian leader, including from influential Arab nations such as Saudi Arabia.
"The future of Syria must be determined by its people, but President Bashar al-Assad is standing in their way. His calls for dialogue and reform have rung hollow while he is imprisoning, torturing, and slaughtering his own people," Mr Obama said in a statement announcing the sanctions.
"For the sake of the Syrian people, the time has come for President Assad to step aside," he added.
The new US sanctions immediately freeze all Syrian government assets subject to US jurisdiction and prohibit any any transactions involving the Assad government.
They also ban US imports of Syrian petroleum or petroleum products - a tiny fraction of US oil imports - and prohibit US citizens from operating or investing in Syria.
The United States has already imposed several rounds of sanctions on Dr Assad and other Syrian officials and financial institutions, but these are believed to have had little impact given the limited nature of US-Syrian dealings.
The United States has long accused Syria of being a state sponsor of terrorism, of meddling in the affairs of Lebanon, supporting Palestinian militants and fomenting violence against US forces in Iraq.
Last week, US secretary of state Hillary Clinton called on countries to stop buying Syrian oil and gas - Europe's main purchasers include France, Germany and The Netherlands - and to get China and India to curb their investments in Syria.
"For months the world has borne witness to the Assad regime's contempt for its own people," Ms Clinton said in a brief State Department appearance today. "The transition to democracy in Syria has begun and its time for Assad to get out of the way."
A source said European nations were likely to impose additional sanctions on Syria, possibly targeting the energy sector, but that these would take more time to prepare.
Dr Assad told UN Secretary-General Ban Ki-moon today that military and police operations against pro-democracy protesters had stopped, the United Nations said.
In a phone call with Dr Assad, Mr Ban "expressed alarm at the latest reports of continued widespread violations of human rights and excessive use of force by Syrian security forces against civilians across Syria, including in the Al Ramel district of Latakia, home to several thousands of Palestinian refugees," the United Nations said in a statement.
"The secretary general emphasised that all military operations and mass arrests must cease immediately. Dr Assad said that the military and police operations had stopped," the statement added.
Residents of the besieged port city of Latakia said yesterday that Syrian forces raided houses in a Sunni district, arresting hundreds of people and taking them to a stadium after a four-day tank assault to crush protests.
Dr Assad's forces attacked al-Raml, a seafront area named after a Palestinian refugee camp built in the 1950s, on the weekend as part of a fierce campaign to crush a five-month-old uprising.
Latakia is of particular significance to Dr Assad, from Syria's minority Alawite community. The president (45), a self-declared champion of the Palestinian cause, comes from a village to the southeast, where his father is buried. The Assad family, along with friends, control the city's port and its finances.
Syria has expelled most independent media since the unrest began, making it difficult to verify reports from the country.
Reuters