President Barack Obama called Thursday for Syrian leader Bashar al-Assad to step down -- significantly ratcheting up international pressure against a regime that has been criticized for its harsh crackdown against anti-government protesters in recent months.
"The future of Syria must be determined by its people, but President Bashar al-Assad is standing in their way," Obama said in a written statement. "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."
U.S. authorities also imposed new economic sanctions against Damascus, freezing Syrian government assets in the United States, barring Americans from making new investments in Syria and prohibiting any U.S. transactions relating to Syrian petroleum products, among other things.
Obama administration officials previously said al-Assad has "lost legitimacy" and that Syria "would be better off" without him. Until now, however, U.S. authorities had resisted calling explicitly for his ouster.
The public call for al-Assad's to step down -- long awaited in many quarters -- was closely coordinated with European, Turkish and Arab allies. It came one day after al-Assad told the head of the United Nations that military and police operations against anti-government protesters have stopped, according to a statement released by the U.N. secretary-general's office.
"These are very strong measures," said Andrew J. Tabler, Syria expert at the Washington Institute for Near East Policy. "These are also unprecedented. We've never targeted Syria energy before. It is the regime's Achilles heel."
Tabler -- also the author of "In the Lion's Den: An Eyewitness Account of Washington's Battle with Syria" -- said no one expects al-Assad "to tip over tomorrow" but he said "these are devastating blows."
He said 90 percent of Syria's oil sales go to European Union countries and the EU is meeting on Friday to do the same thing that the United States has done.
The leaders of France, Germany and the United Kingdom joined Obama Thursday in calling on al-Assad to step down.
"Our three countries believe that President Assad, who is resorting to brutal military force against his own people and who is responsible for the situation, has lost all legitimacy and can no longer claim to lead the country," British Prime Minister David Cameron, French President Nicolas Sarkozy and German Chancellor Angela Merkel said in a statement.
"We call on him to face the reality of the complete rejection of his regime by the Syrian people and to step aside in the best interests of Syria and the unity of its people."
U.S. Secretary of State Hillary Clinton said the United States "will take steps to mitigate any unintended effects of the (new) sanctions on the Syrian people."
Clinton also said the United States expects that other countries "will amplify the steps we are taking."
U.S. officials, who asked not to be named because of the sensitivity of the matter, have said they expect similar calls for al-Assad to step down in coming days from other leaders.
Senior U.S. officials, diplomats and members of the Syrian opposition have argued that the next several weeks will be critical in terms of isolating al-Assad, strengthening sanctions against the regime, and bolstering domestic opponents of the regime.
The campaign against al-Assad, involving intense diplomatic outreach by Clinton and Obama himself, has been based on the one the United States used in Libya, where the Obama administration built international consensus for the NATO mission to protect civilians.
While there has been little talk of military intervention in Syria, Clinton and Obama have been working with European, Turkish and Saudi leaders to coordinate tougher diplomatic and economic action against al-Assad.
On Thursday, the U.N. Security Council will hold consultations on Syria, when they will be briefed by the organization's High Commissioner for Human Rights Navi Pillay.
Officials said Pillay is expected to give a sober assessment of the situation on the ground in Syria.
France and other European countries are continuing to push for a Security Council resolution condemning the al-Assad regime, but they are still facing stiff resistance from Russia, China, Brazil, India and South Africa. The United States is pushing for special session on Syria at the U.N. Human Rights Council in Geneva, Switzerland, as early as next week.
Diplomats say they hope Pillay's briefing Thursday will generate momentum for further action, including widespread calls for al-Assad to step down over the coming days, and build consensus toward the Security Council resolution, which could serve as a pretext for further action.