WASHINGTON (AP) -- In a choreographed diplomatic squeeze play, President Barack Obama, the leaders of Britain, France and Germany and the European Union joined Thursday to demand that Syrian President Bashar Assad resign, saying his brutal suppression of his people had made him unfit to lead.

The coordinated messages from Washington, London, Paris, Berlin and Brussels were accompanied by a U.N. recommendation that Syria be referred to the International Criminal Court for investigation of atrocities and by tough new U.S. sanctions freezing all Syrian government assets in the U.S. and targeting the country's lucrative energy sector.

The moves intensified already mounting pressure on Assad, who has refused to ease his regime's ruthless crackdown on a five-month old opposition uprising and has backed away from promises of reform. Instead, he has unleashed his security forces on numerous cities, killing nearly 2,000 people, many of them innocent civilians, according to rights groups.

The resignation calls were the first explicit demands from the U.S. and its allies for Assad to step down although condemnation of his actions had been mounting for weeks since the regime ordered a sustained assault on its opponents on the eve of the Muslim holy month of Ramadan that began in early August.

In a written statement released by the White House, Obama said Assad had lost all credibility as a leader and had to go.

"His calls for dialogue and reform have rung hollow while he is imprisoning, torturing, and slaughtering his own people," Obama said. "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."

"It is clear that President Assad believes that he can silence the voices of his people by resorting to the repressive tactics of the past. But he is wrong," Obama said after signing an executive order that gives his administration authority to impose sweeping new sanctions on Syria intended to further isolate the regime.

The order immediately bans the import into the United States of any Syrian petroleum or petroleum products. Syria is not a huge source of oil for the U.S., but if European allies join the effort, it could significantly affect one of the regime's top sources of revenue. Syrian crude oil exports go mostly to European countries such as Germany, Italy, and France, according to the U.S. Energy Information Agency.

Obama's order also denies Syria access to the US financial system, freezing all Syrian government assets that are subject to U.S. jurisdiction. It also prohibits any U.S. citizen from engaging in transactions with the regime, investing in the country or exporting services to Syria. Under the order, the Treasury Department also could impose sanctions against persons who give support to Assad's government.

American officials acknowledged that the U.S. moves alone will not likely have any immediate impact on the Syrian regime's behavior but they stressed that it along with the other calls send a powerful signal that Assad is no longer welcome in the international community.

The sanctions "further tighten the circle of isolation" and "strike at the heart of the regime," Secretary of State Hillary Rodham Clinton said at the State Department.

The U.S. had already hit more than 30 Syrian regime officials, including Assad himself and members of his inner circle, and firms with sanctions. It has also lobbied other nations to take similar steps, something Clinton predicted would come soon.

But the administration was careful to try not to appear highhanded or meddlesome in a region where suspicion of U.S. motives is rampant. Some of the Syrian protesters demanding an end to the Assad regime also reject any new alliance with the American government. Clinton and Obama stressed no one would impose transition in Syria.

"We understand the strong desire of the Syrian people that no foreign country should intervene in their struggle, and we respect their wishes," Clinton said. "At the same time, we will do our part to support their aspirations for a Syria that is democratic, just and inclusive, and we will stand up for their universal rights and dignity by pressuring the regime and Assad personally."

As Syrian protesters have called for an end to his regime, Assad has unleashed tanks and ground troops in an attempt to retake control in rebellious areas. The military assault has escalated dramatically since Ramadan began, with security forces killing hundreds and detaining thousands.

Until Thursday, the administration had said Assad had lost his legitimacy and that Syria would be better off without him. But it had stopped short of demanding his departure. Officials said the administration had waited in order to rally an international consensus behind the call.

The White House had planned to make the announcement last week but postponed it largely at the request of Syria's neighbor Turkey, which asked for more time to try to convince Assad to reform, and because Clinton and other officials argued it was important to build a global coalition to demand his departure.

Clinton on Tuesday had publicly questioned the effectiveness of the United States acting alone.

Since then, however, the coordination strategy clearly bore fruit.

European Union foreign policy chief Catherine Ashton said in a statement Thursday that "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."

At the same time, British Prime Minister David Cameron, French President Nicolas Sarkozy and German Chancellor Angela Merkel issued a joint statement saying that Assad should "leave power in the greater interests of Syria and the unity of his people."

Earlier Thursday, a high-level U.N. human rights team in Geneva said that Syria's crackdown "may amount to crimes against humanity" and should be referred to the International Criminal Court. U.N. High Commissioner for Human Rights Navi Pillay is expected on Thursday afternoon to urge the U.N. Security Council to make that referral.

The investigators say they found "a pattern of human rights violations that constitutes widespread or systematic attacks against the civilian population." In their report, they said they had compiled a confidential list of 50 alleged perpetrators at "various levels" of Assad's government. Syria insists it is rooting out terrorists but rights groups accuse Syrian troops of killing more than 1,800 civilians since mid-March.

Jordan's foreign minister said Thursday that his country is "angered" and "extremely worried" by the killings of civilians in Syria and Switzerland recalled its ambassador. A day earlier, Tunisia recalled its ambassador from Syria, following the lead of several other Arab nations, including Saudi Arabia, that the U.S. has been lobbying to show displeasure with Assad.

Turkish Prime Minister Recep Tayyip Erdogan on Wednesday compared Assad to Libya's Moammar Gadhafi for refusing to heed calls to change. Turkey has joined calls for Gadhafi to leave power and Erdogan said Wednesday he had personally spoken to Assad and sent his foreign minister to Damascus, but "despite all of this, they are continuing to strike civilians."

In New York on Wednesday, U.N. Secretary-General Ban Ki-moon spoke to Assad demanding the immediate end of all military operations and mass arrests. In response, Assad said military and police operations had stopped, according to a U.N. statement said.

But the London-based Syrian Observatory for Human Rights, which documents anti-regime protests, said Thursday that Syrian troops had shot dead nine people in the central city of Homs on Wednesday night. Another rights group said Assad's crackdown also killed nine people elsewhere in Syria on Wednesday.

---

AP White House Correspondent Ben Feller and Associated Press writer Jim, Kuhnhenn contributed to this report.

© 2011 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed. Learn more about our Privacy Policy and Terms of Use.