“We have consistently said that President Assad must lead a democratic transition or get out of the way,” Mr. Obama said in a written statement released Thursday morning after coordination with allies in Europe . “He has not led. For the sake of the Syrian people, the time has come for President Assad to step aside.”

Almost simultaneously, Prime Minister David Cameron of Britain , President Nicolas Sarkozy of France and Chancellor Angela Merkel of Germany issued a joint statement urging Mr. Assad “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.” Canada made a similar appeal, as did the European Union . The United Nations human rights office in Geneva issued a damning, 22-page report that concluded that Syrian government forces might have committed crimes against humanity by carrying out summary executions, torturing prisoners and harming children.

The United Nations report, overseen by the high commissioner for human rights, Navanethem Pillay, accused Syria of grossly violating its citizens’ rights and carrying out “numerous summary executions, including 353 named victims.” It also said that members of the security forces “posed as civilians in order to cause unrest and portray an inaccurate picture of events.”

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The office recommended that the United Nations Security Council consider referring Syria to the International Criminal Court for prosecution. In New York, the Security Council met later on Thursday and discussed potential steps against Syria.

The United States and European members of the Security Council pressed for a resolution condemning Syria and were debating sanctions that could include an arms embargo, a freezing of assets and a ban on travel by the country’s leaders, diplomats there said. They would not say whether the Council would consider referring the matter to the international court, but did say that Council members expressed a desire to hold accountable those responsible for the violence. At the United Nations, Syria said the United States was trying to use the Security Council as an “instrument” to instigate further instability.

Even Russia , which has resisted punitive measures against Syria so far and appears likely to veto an embargo, has sounded increasingly frustrated with Mr. Assad’s government, which has ignored repeated calls to halt the violence, including those from countries like Turkey with which it had closer relations. While Mr. Assad told the United Nations secretary general, Ban Ki-moon , on Wednesday that the crackdown had in fact ended, activists within Syria said the violence continued unabated.

After Mr. Assad spoke on Wednesday, two people were reported killed in the city of Homs after a nighttime prayer, held only during the month of Ramadan , when Muslims fast from dawn to sunset; at least one more died in the city on Thursday when security forces opened fire on a small protest in the neighborhood of Baba Amr. Residents and activists also reported attacks and arrests in other cities, including Deir al-Zour, Hama and the capital, Damascus. “The killings and destruction haven’t stopped,” an activist in Homs, who would be identified only as Mohamad, said in a telephone interview on Thursday. “They haven’t withdrawn from the city. On the contrary, the number of security forces has increased and gunfire is heard every so often.”

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In Washington, administration officials acknowledged that American sanctions alone would have little effect. “No outside power can or should impose this transition,” Secretary of State Hillary Rodham Clinton said at the State Department. “It is up to the Syrian people to choose their own leaders, in a democratic system based on the rule of law and dedicated to protecting the rights of all citizens, regardless of ethnicity, religion, sect or gender.”

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With Syria’s opposition barely organized and under constant harassment, there is no obvious path to a transfer of power, even if Mr. Assad’s grip weakens significantly. “Nothing about this is going to be easy,” a senior administration official said.

The sanctions that Mr. Obama ordered took aim at Syria’s oil and petroleum exports. Syria is a relative small energy producer, exporting only 148,000 barrels of oil a day, almost all of it to European nations, according to the United States Department of Energy . Mr. Cameron, Mr. Sarkozy and Ms. Merkel said in their statement that they would be “actively supporting” additional sanctions through the European Union, but no new ones were announced Thursday. Like the United States, the European Union has imposed sanctions on Mr. Assad and dozens of other security officials and businessmen in Syria. The union’s foreign policy chief, Catherine Ashton , said the union was discussing how to broaden its sanctions.

The Obama administration has sharply criticized the violence in Syria from the start, steadily intensifying its pressure on Mr. Assad with statements and with actions. But in stark contrast to its comparatively swift calls for the ouster of Egypt ’s president, Hosni Mubarak , or Libya ’s leader, Col. Muammar el-Qaddafi , the administration stopped short of doing so in the case of Syria.

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The White House and State Department debated the issue intensely in recent weeks, with Mrs. Clinton arguing that a call for Mr. Assad to go would be meaningless without broader support and action. An administration official said the president first broached calling for Mr. Assad’s departure in a phone call with Mr. Sarkozy and Ms. Merkel on Aug. 5.

Mrs. Clinton persuaded the White House to delay its call until the sanctions could be readied and to give Turkey’s foreign minister, Ahmet Davutoglu, a chance to press the Syrian leader to halt the violence and begin a reform process that would include a unity government and new elections. As the violence continued and intensified with the start of Ramadan, Mr. Obama spoke with Turkey’s prime minister, Recep Tayyip Erdogan , and said he wanted to move more quickly, an administration official said.

Turkey, with a shared border and deep historic and economic ties to Syria, did not join in the diplomatic chorus on Thursday. “Turkey was the first country to say that there was no point in continuing talks with Syria while military operations continued,” said a Turkish government official, who asked not to be named because of his diplomatic position. “Turkey, however, is not at the point mentioned in President Obama’s speech today.”