US President Barack Obama said Thursday that Syria's leader Bashar al-Assad must allow a political transition or step aside, as activists readied to stage fresh pro-democracy protests despite the regime's brutal crackdown.
Obama's call, in a major speech on the Middle East, represented a further stiffening of the US line and came a day after Washington slapped new sanctions against Assad and six of the Syrian president's top aides.
"The Syrian people have shown their courage in demanding a transition to democracy," Obama said in a key policy speech responding to the events of the "Arab Spring".
"President Assad now has a choice: He can lead that transition, or get out of the way.
"The Syrian government must stop shooting demonstrators and allow peaceful protests," said the US leader.
"It must release political prisoners and stop unjust arrests. It must allow human rights monitors to have access to cities like Daraa and start a serious dialogue to advance a democratic transition.
"Otherwise, President Assad and his regime will continue to be challenged from within and will continue to be isolated abroad," warned Obama.
For more than two months, pro-democracy protests have challenged Assad's regime, which has retaliated with deadly force, sweeping arrests and torture, rights activists say.
Obama said the United States had imposed sanctions against Assad and those around him on Wednesday after Damascus chose "the path of murder and the mass arrests of its citizens."
Assad's government denounced the sanctions, saying they were part of long-time efforts by Washington to serve Israeli interests.
"The US measures are part of a series of sanctions imposed by successive US administrations against the Syrian people as part of a regional scheme, aimed primarily at serving Israel's interests," SANA news agency said.
"Any aggression against Syria is akin to US support for Israeli aggressions against Syria and the Arabs," the state news agency said.
Sanctions "have not and will not" affect decisions taken by the Syrian government or its struggle against US hegemony.
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Meanwhile, Facebook group The Syrian Revolution 2011 called for nationwide demonstrations on Friday for "liberty and national unity."
It said the protests should include Kurdish towns in the north, labelling the day "Friday of Freedom, Azadi," or liberty in Kurdish.
More than 850 people are believed to have been killed and another 8,000 arrested since the protests began, according to rights groups and the United Nations.
Syria blames "armed gangs" for the bloodshed.
Adding to claims of torture was Al-Jazeera journalist Dorothy Parvaz, who arrived in Qatar on Wednesday from Iran after having been held in a Syrian prison where, she said, she heard the cries of people being tortured.
"I was in a Syrian detention centre for three days, two nights, and what I heard were just savage beatings," she said Thursday.
France, which has been pushing for a United Nations Security Council resolution condemning repression in Syria, urged Damascus to withdraw troops deployed in cities and towns.
In imposing the sanctions, the US administration stopped short of saying Assad had lost his legitimacy to rule, a formula Washington has applied to Libyan leader Moamer Kadhafi, whose forces are fighting an armed rebellion.
The sanctions allow Washington to freeze any assets owned in the United States by Assad and his top aides and ban any individuals or US companies from dealing with them. But it is unclear what assets would be blocked.
Sanctions were also slapped on Syrian Vice President Faruq al-Shara, Prime Minister Adel Safar, Interior Minister Mohammed Ibrahim al-Shaar, Defence Minister Ali Habib Mahmud, military intelligence chief Abdul Fatah Qudsiya and Mohammed Dib Zaitoun, director of the Political Security Directorate.
Also named were Ali Mamluk, director of Syria's Intelligence Directorate, and Atif Najib, the ex-head of intelligence in Daraa province, epicentre of protests that has gripped Syria since March 15.
The US Treasury also imposed sanctions on Syria's Military Intelligence, National Security Bureau and the Air Force Intelligence, as well as on Hafiz Makhluf, a cousin of Assad.
The European Union has slapped sanctions on members of Assad's inner circle and said it is also contemplating targeting the president, while France is trying to get support for a UN resolution condemning Syria.
UN chief Ban Ki-moon told AFP Wednesday he had been urging Assad to carry out reforms "before it is too late."