President Barack Obama Thursday led a chorus of calls by world leaders for Syrian President Bashar al-Assad to step down, as the UN warned his regime could be guilty of crimes against humanity.

Syrian President Bashar al-Assad (C), new Defence Minister Hassan Turkmani (L) and former Defence Minister Mustafa Tlas (R) attend a ceremony at the unknown soldier monument in Damascus in this October 6, 2003 file photo. President Barack Obama on August 18, 2011 for the first time called on Syrian President al-Assad to step down after a brutal crackdown by Syrian forces on demonstrations against the Assad family’s 41-year reign. REUTERS/Sana/Files

Obama also slapped harsh new sanctions on Syria in an executive order that freezes all Syrian government assets and forbids investment and exports to the country.

In a statement accompanying the order, Obama said Syria’s violent crackdown on anti-regime protests “constitute an unusual and extraordinary threat to the national security, foreign policy, and economy of the United States.”

It was the first explicit U.S. 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.

“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.

His call was quickly echoed by President Nicolas Sarkozy of France, Germany’s Chancellor Angela Merkel and British Prime Minister David Cameron.

“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,” they said in a joint statement.

The European Union too joined the groundswell of calls for Assad to go.

“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.

U.S. Secretary of State Hillary Clinton later emphasized that U.S. sanctions on Syrian oil “strike at the heart of the Syrian regime.”

“These actions strike at the heart of the regime by banning American imports of Syrian petroleum and petroleum products and prohibiting Americans from dealing in these products,” Clinton said in a statement read out to reporters.

Earlier on Tuesday she said that Turkey, Saudi Arabia and other governments should call on Syrian President Bashar al-Assad to step down, but declined to make that call herself.

“It’s not going to be any news if the United States says Assad needs to go,” Clinton said, suggesting the world’s reaction to such a move would be, “Ok, fine. What’s next?”

“If Turkey says it, if King Abdullah (of Saudi Arabia) says it, if other people say it, there’s no way the Assad regime can ignore it,” Clinton said during an appearance with Defense Secretary Leon Panetta at the US National Defense University.

On Thursday, UN human rights chief Navi Pillay 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.

A report by Pillay described a “pattern of human rights violations that constitutes widespread or systematic attacks against the civilian population, which may amount to crimes against humanity.”

And the report urged the UN Security Council, which was to meet later on Thursday, to “consider referring the situation in Syria to the International Criminal Court.”

It described a “widespread modus operandi to kill civilians by using a) forces on the ground, b) snipers on rooftops and c) air power … consistent with an apparent shoot-to-kill policy.”

The document also describes summary executions, including reports that “forces conducted regular raids in hospitals to search for and kill injured demonstrators,” as well as allegations of torture and arbitrary arrests.

It urged an immediate end to “the gross human rights violations, including the excessive use of force against demonstrators and the killing of protesters, torture and ill-treatment of detainees and enforced disappearance.”

And on Monday, the UN Human Rights Council is to hold a special session on Syria requested by 24 members of the council, including European Union members, Washington and all four Arab members — Jordan, Kuwait, Qatar and Saudi Arabia.

The tide of international outrage came even as Assad announced his security forces had ended their deadly crackdown on dissent.

Activists in Syria confirmed that the situation appeared to be calm for the first time in weeks, but called for fresh mass protests on Friday after the weekly day of Muslim prayers.

Deputy UN spokesman Farhan Haq said Assad spoke to UN Secretary General Ban Ki-moon by telephone.

Ban told him he was alarmed “at the latest reports of continued widespread violations of human rights and excessive use of force by Syrian security forces against civilians across Syria.”

He “emphasized that all military operations and mass arrests must cease immediately. President Assad said that the military and police operations had stopped,” according to the spokesman.

On the ground, the head of the Syrian Observatory for Human Rights, Rami Abdel Rahman, said after contacts with activists in several towns across Syria that the situation was calm on Thursday.

“I have not heard of any military action until now, apart from gunshots heard in (the port city of) Latakia,” he said in Nicosia by telephone. “But the situation could change at any moment.”

According to rights groups, the crackdown on dissent in Syria has killed 1,827 civilians, while 416 security forces have also died in what the authorities have said was a campaign against terrorists and armed gangs. 

A defiant Assad on Wednesday told his ruling Baath party that Syria would “remain strong and resilient” despite international pressure, adding he had promised reforms “because Syrians were convinced of their necessity.”

Since April, the embattled president has tried to quell the growing protests. He has lifted a state of emergency in force since 1963 and authorised political parties alongside the Baath.

A key demand of the opposition has been the removal of Article 8 of the constitution which stipulates that the Baath party is the sole “leader of state and society,” giving it a monopoly on power.

But in defiance of growing international condemnation, hundreds of Syrian security services raided homes in the Mediterranean port city of Latakia on Wednesday, activists said.

In the central city of Homs, security forces killed nine people, including two gunned down outside a mosque and a civilian was shot dead by a sniper in its Armenian neighbourhood, activists said.

In Idlib province near the border with Turkey, security forces shot dead a man standing on his balcony, the Britain-based Syrian Observatory for Human Rights said.

Arab states along with European countries and the United States are to ask the United Nations’ top human rights body to hold a special session on Syria, diplomats in Geneva said.

The United Nations said on Wednesday it has withdrawn about 25 international staff and dozens of families of expatriate workers from Syria due to mounting security fears.

The United States, meanwhile said travel restrictions have been placed on Syrian diplomats living in Washington, in a tit-for-tat measure after a similar move by Damascus earlier this month.

According to rights groups, the crackdown on dissent in Syria has killed 1,827 civilians, while 416 security forces have also died in what the authorities have said was a campaign against terrorists and armed gangs.