US President Barack Obama demanded Thursday that Syrian President Bashar al-Assad "step aside" and imposed tough sanctions on Damascus including an asset freeze and ban on US investments in Syria.
"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," Obama said.
It was the first explicit US call for Assad to resign as global pressure increased on the Syrian leader to end a months-long crackdown on dissent that has killed more than 2,000 people, according to rights activists.
But Obama also emphasized that Washington "cannot and will not impose this transition upon Syria" and vowed to heed Syrians' "strong desire that there not be foreign intervention in their movement."
"It is time for the Syrian people to determine their own destiny, and we will continue to stand firmly on their side," Obama said in a stern statement quickly echoed by the European Union.
"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," foreign policy chief Catherine Ashton said in a statement.
Earlier Thursday, the UN human rights chief said Syria may have committed crimes against humanity in its bloody crackdown on dissent and urged the UN Security Council to refer the matter to the International Criminal Court.
The UN Security Council was due to meet later Thursday to hear from that official, Navi Pillay.
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The US president imposed a wave of new economic sanctions on Syria that he said would "deepen the financial isolation of the Assad regime," already targeted for being a US-designated state sponsor of terrorism.
Obama ordered Syrian government assets under US jurisdiction frozen, blocked any US person or business from doing business with Syria's government, prohibited US imports of petroleum or petroleum products from Syria, and banned US individuals or firms from "operating or investing in Syria."
"We expect today's actions to be amplified by others," said the US president, who noted that any Syrian democratic transition "will take time" and warned the Syrian people faced "more struggle and sacrifice."
The United States had previously said Assad had lost his legitimacy to govern and said Syria would be better off without him but had not explicitly demanded he quit power.
But activists and some in Washington had criticized Obama for calling on Libyan strongman Moamer Kadhafi to step aside but stopping short of doing so with Assad, whose bloody crackdown on dissent has drawn global outrage.
US officials have said for the past week that the administration was planning to call on Assad to step down but had not yet decided on the timing.
US Secretary of State Hillary Clinton has indicated the United States did not want to go out on a limb and that it was essential that Syria's neighbors and the international community simultaneously increase pressure on Assad.
Clinton has said that Arab pressure on Syria was particularly important in the campaign against Assad. This week Tunisia recalled its ambassador from Damascus, following similar moves by Saudi Arabia, Kuwait, and Bahrain.
The developments at the United Nations came after a special session of the UN human rights council which diplomats said was requested by not only the United States and EU but also all four Arab countries that are members of the council -- Jordan, Kuwait, Saudi Arabia and Qatar.