The United States and its European allies have called on Syria's leader to step down. And U.S. President Barack Obama accused him of “torturing and slaughtering” his own people in what U.N. officials say could be crimes against humanity.

Mr. Obama released a statement Thursday saying the time had come for Syrian President Bashar al-Assad to “step aside.” He also ordered Syrian government assets in the U.S. frozen, banned U.S. citizens from operating in or investing in Syria and prohibited U.S. imports of Syrian oil products.

EU foreign policy chief Catherine Ashton also called on Mr. Assad to resign, saying he has completely lost legitimacy in the eyes of his people. She said the EU is preparing to broaden sanctions against Syria. Britain, France, Germany and Canada also urged Mr. Assad to step down.

In New York, the U.S. and four European members of the United Nations Security Council said they would begin drafting a U.N. sanctions resolution against Syria. But it could be a lengthy and difficult process to persuade council members Russia and China not to veto such a measure.

Syria's U.N. ambassador, Bashar Ja'afari, accused Washington and some other Security Council members of waging what he called a “diplomatic and humanitarian war” against his country.

In a report released Thursday, U.N. investigators said Mr. Assad's forces had carried out widespread and systematic attacks on civilians, including more than 350 summary executions while using snipers, air power and other military measures. The report said pro-government troops had tortured anti-government protesters and targeted children in the crackdown, which it said “may amount to crimes against humanity.”

U.N. human rights chief Navi Pillay told the council it should refer the situation in Syria to the Hague-based International Criminal Court. The world body plans to send a team to Syria in a few days to assess the humanitarian situation there.

On Wednesday, U.N. Secretary-General Ban Ki-moon told Mr. Assad he was alarmed at reports of the 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.

Much of Syria was quiet Thursday. But activists said 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 more than 700 troops raided homes in Latakia's southern al-Raml district, arresting people whose names appeared on lists.

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

U.N. investigators say at least 1,900 civilians have been killed since the start of the government's crackdown in mid-March and that body wounds were “consistent with an apparent shoot-to-kill policy.”