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Hillary Clinton: "The transition to democracy in Syria has begun, and it is time for Assad to get out of the way"
The leaders of the US, UK, France, Germany and the EU have all called for Syria's President Assad to step down over his suppression of protesters.
US President Barack Obama said: "The time has come for President Assad to step aside."It marks a significant increase in pressure on Mr Assad for sending in his army against the protesters.
Meanwhile, UN investigators say the use of violence in Syria "may amount to crimes against humanity".
In a report to the UN Human Rights Council, the investigators said the UN Security Council should refer the issue to the International Criminal Court.
Human rights groups believe about 2,000 people have been killed and thousands arrested since March as Syria's security forces - including tanks, helicopters, gunships and snipers - try to quell dissent that has broken out in much of the country.
President Bashar al-Assad has promised political reforms but blames the unrest on "terrorist groups".
'Get out of the way' In a written statement, Mr Obama said: "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.
He added: "We have consistently said that President Assad must lead a democratic transition or get out of the way. He has not led. For the sake of the Syrian people, the time has come for President Assad to step aside."
Mr Obama also announced "unprecedented sanctions to deepen the financial isolation of the Assad regime and further disrupt its ability to finance a campaign of violence against the Syrian people".
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Addressing a large gathering of Baath Party faithful shortly before the US-European move, Mr Assad told them that Syria would stick to its "nationalist, resisting positions" however much outside pressures might mount.
Hostility from the US and the West in general is a given as far as Damascus is concerned, so its reaction is bound to be defiant.
But President Obama's statement and the comments from Secretary of State Clinton are unlikely to be directly reported by the Syrian state media, and Syrian leaders may try to play them down by not reacting directly to them.
The same goes for the UN report to the Security Council on human rights abuses in Syria.
If past form is any guide, the Syrian reaction will more likely take the form of quoting adverse comments from allies or sympathisers without explaining exactly what they are referring to.
Analysis
Hostility from the US and the West in general is a given as far as Damascus is concerned, so its reaction is bound to be defiant.
But President Obama's statement and the comments from Secretary of State Clinton are unlikely to be directly reported by the Syrian state media, and Syrian leaders may try to play them down by not reacting directly to them.
The same goes for the UN report to the Security Council on human rights abuses in Syria.
If past form is any guide, the Syrian reaction will more likely take the form of quoting adverse comments from allies or sympathisers without explaining exactly what they are referring to.
The US had already tightened its sanctions against members of Syria's government but had stopped short of demanding Mr Assad step down.
At the same time, the EU and the leaders of Britain, France and Germany issued statements also calling on President Assad to leave.European Union foreign policy chief Catherine Ashton said: "The EU notes the complete loss of Bashar al-Assad's legitimacy in the eyes of the Syrian people and the necessity for him to step aside."
In a joint statement, the leaders of Britain, France and Germany said President Assad should "leave power in the greater interests of Syria and the unity of his people".
Canadian Prime Minister Stephen Harper joined the other leaders in calling on "President Assad to vacate his position, relinquish power and step down immediately".
A Syrian government spokesman accused Western governments of increasing the tension in the country.
"It is strange that instead of offering [Damascus] a helping hand to implement its programme of reforms, the West and Obama are seeking to stoke more violence in Syria," Reem Haddad, of the information ministry, told AFP news agency.
'Systematic attacks' The new sanctions announced by Mr Obama included a freeze on all assets of the Syrian government subject to US jurisdiction and a ban on US citizens from carrying out any transactions with the Syrian government.
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US sanctions against Syria
- Syrian government assets frozen
- New investment banned
- Imports of Syrian petroleum banned
- Assets of 32 Syrian and Iranian individuals frozen and dealings with US citizens prohibited, including President Assad, his brother Maher and other government officials
US imports of Syrian petroleum or petroleum products are banned and US citizens are prohibited from having any dealings related to Syria's petroleum industry.
US citizens are also prohibited from "operating or investing in Syria", Mr Obama's executive order says.US Secretary of State Hillary Clinton said the sanctions would "further tighten the circle of isolation" and "strike at the heart of the regime".
The calls for Mr Assad to step down follow a report from UN investigators into the recent violence in Syria.
Their 22-page report says that security forces, including snipers, have used deadly force against civilians in attempts to quell months of anti-government protests.
The UN's investigators were not allowed in Syria. They interviewed victims and witnesses of the violence, some in Syria, and others in the region.
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The BBC's Jim Muir in Beirut says activists continue to report deaths and "the repression goes on"
The report, released in Geneva, urged the UN Security Council to "consider referring the situation in Syria to the International Criminal Court".
The UN High Commissioner for Human Rights, Navi Pillay, is to brief the Security Council on the report later on Thursday in a special session.
An emergency session of the UN Human Rights Council will be held on Monday following a request from all 24 members - including Jordan, Kuwait, Qatar and Saudi Arabia.
Despite earlier assurances from Mr Assad to UN chief Ban Ki-moon that military operations against protesters had stopped, activists in Syria said at least 20 people were killed around the country on Wednesday, and gunfire in the city of Homs continued overnight.
The unrest in Syria began in March following the toppling of Tunisian President Zine al-Abidine Ben Ali and Egyptian President Hosni Mubarak.
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