The United States and other world powers are calling for the resignation of Syrian President Bashar al-Assad because of his deadly crackdown on political protesters.

U.S. President Barack Obama issued a statement Thursday saying the time had come for Mr. Assad to “step aside.” He released an executive order announcing “unprecedented sanctions” on Syria, in a bid to deepen the economic isolation of Mr. Assad's government. Britain, France, Germany and the European Union also urged Mr. Assad to step down.

Shortly after the White House statement was released, U.S. Secretary of State Hillary Clinton said the transition to democracy had begun in Syria and that it was time for Mr. Assad to “get out of the way.”

Mr. Assad has been facing increasing international pressure because of his brutal crackdown on anti-government dissent. But Thursday marked the first time the U.S. had formally called for his resignation.

The U.S. change of position on Syria comes on the same day the U.N. Security Council is to hear a report on the Syrian government's tactics. U.N. human rights investigators say the crackdown on dissent may amount to “crimes against humanity.”

The findings are in a 22-page report that was released by the office of U.N. human rights chief, Navi Pillay, ahead of her briefing to the Security Council. Pillay is expected to call for the Security Council to refer Syria to the International Criminal Court for alleged human rights violations.

The report from her office cites “widespread” attacks against civilians that include the use of snipers, air power and other military measures.

On Wednesday, U.N. Secretary-General Ban Ki-moon told Mr. Assad that he was alarmed at reports of excessive use of force by government security forces and continued widespread violations of human rights.

Mr. Ban spoke by telephone with Mr. Assad, who said military and police operations had stopped.

Activists said Syrian security forces detained hundreds of people Wednesday in the besieged port city of Latakia, where at least 35 people have been killed during the past week.

A London-based Syrian rights group said that more than 700 troops raided homes in Latakia's southern al-Raml district, arresting people on lists.

Activists also said forces loyal to Mr. Assad killed nine people in the central city of Homs, including two protesters shot dead in front of a mosque after nightly Ramadan prayers.

Latakia's al-Raml area is home to a crowded Palestinian refugee camp where many low-income Syrians also live. U.N. officials say as many as 10,000 residents fled the neighborhood during the government's four-day operation to crush dissent in the city.

The United Nations Relief and Works Agency said Wednesday it has determined the whereabouts of about 2,000 of the displaced people and is providing them with aid.

Rights groups and activists say at least 1,800 civilians have been killed since the start of the government's crackdown in mid-March.