The United States and European Union called on Syrian President Bashar al-Assad to step down on Thursday and President Barack Obama accused him of torturing and slaughtering his own people in what U.N. officials said could be crimes against humanity.

It was a dramatic sharpening of international rhetoric -- major states had urged Assad to reform rather than resign.

But with no threat of Western military action like that against Libya's Muammar Gaddafi, the five-month-old conflict between Assad and his opponents seems likely to grind on in the streets.

Putting faith in sanctions rather than force, Obama ordered Syrian government assets in the United States frozen, banned U.S. citizens from operating in or investing in Syria and prohibited U.S. imports of Syrian oil products.

Though U.N. Secretary-General Ban Ki-moon said Assad had assured him on Wednesday that military operations were over, activists said Syrian forces carried out further raids in Deir al-Zor and surrounded a mosque in Latakia on Thursday.

The future of Syria must be determined by its people, but President Bashar al-Assad is standing in their way, Obama said. His calls for dialogue and reform have rung hollow while he is imprisoning, torturing and slaughtering his own people.

In a coordinated move, European Union foreign policy chief Catherine Ashton called on Assad to step aside and said the EU was preparing to broaden sanctions against Syria.

At the United Nations, Britain, France, Portugal, Germany and the United States said they would begin drafting a Security Council sanctions resolution on Syria. We believe that the time has come for the council to take further action, Britain's Deputy U.N. Ambassador Philip Parham told reporters.

U.N. human rights investigators said Assad's forces had carried out systematic attacks on civilians, often opening fire at close range and without warning, and committing violations that may amount to crimes against humanity.

A U.N. report issued in Geneva recounted complaints of indiscriminate shooting and of wounded people being put to death with knives or by being dumped in the refrigerated rooms of hospital morgues.

CRACKDOWN CONTINUING

The Security Council should consider referring the situation in Syria to the Hague-based International Criminal Court, the investigators said, adding that they had evidence against 50 suspects.

U.N. human rights chief Navi Pillay conveyed that message in a briefing to the council in New York later in the day. I ... recommended referral to the ICC, she told reporters. Diplomats said they would study the recommendation.

In a telephone call with Assad on Wednesday U.N. Secretary General Ban joined a chorus of condemnation, expressing alarm at reports of widespread violations of human rights and excessive use of force by security forces against civilians.

The Syrian Revolution Coordinating Union, an activists' group, said security forces fired machineguns near a mosque in Latakia which was surrounded by armored vehicles.

It also said Assad's forces killed at least one man when they fired live ammunition to stop residents from marching after Ramadan prayers in the Mureijeh neighborhood of Homs, 165 kilometers (100 miles) north of the capital Damascus.

Separately, it said security forces shot dead a man it identified as Ali al-Hussein and wounded six when they fired at a sit-in in the town of al-Ruhaibeh northeast of Damascus.

In New York, Syria's U.N. Ambassador Bashar Ja'afari accused the United States and European powers of waging a diplomatic and humanitarian war against Syria by imposing sanctions and demanding that Assad quit.

Britain's Parham told reporters the proposed U.N. sanctions would include an arms embargo, asset freezes and travel bans. But veto-holders Russia and China have so far opposed concrete measures against Damascus.

SANCTIONS IMPACT

Nadim Shehadi of London's Chatham House think-tank said the shift in tone from Washington and Europe was significant, since it may encourage Syrians who saw previous calls for Assad to reform as an indication of support for him.

The previous messages from the West to Bashar al-Assad were ambiguous, Shehadi said. Now the West has hit at the very basis of the idea of his power, by telling him that we don't believe in you any more and you should leave.

Rosemary Hollis, Middle East politics lecturer at London's City University, said the move could rattle the regime, they will feel very isolated.

It may take time, however, for the diplomatic broadside, backed by sanctions, to have an impact on the 45-year-old president who took power when his father President Hafez al-Assad died 11 years ago after three decades in office.

He has so far brushed off international pressure and survived years of U.S. and European isolation following the 2005 assassination of Lebanese statesman Rafik al-Hariri, a killing many Western nations held Damascus responsible for.

Despite the escalating international rhetoric and Western sanctions, no country is proposing to take the kind of military action NATO forces launched in Libya to support rebels fighting Gaddafi. That action has helped them take much of the country.

However, Syria's economy, already hit by a collapse in tourism revenue, could be further damaged by Obama's announcement. U.S. sanctions will make it very difficult for banks to finance transactions involving Syrian oil exports.

It will also make it challenging for companies with a large U.S. presence, such as Shell to continue producing crude in Syria -- although the impact on global oil markets from a potential shutdown of Syria's 380,000 barrels per day oil industry would be relatively small compared to Libya.

Assad says the protests are a foreign conspiracy to divide Syria and pledged last week his army would not relent in pursuing terrorist groups.

U.N. INVESTIGATION

The U.N. investigators said Syrian forces had fired on peaceful protesters throughout the country, often at short range and without warning, killing at least 1,900 civilians, including children. Their wounds were consistent with an apparent shoot-to-kill policy, their report said.

The mission found a pattern of human rights violations that constitutes widespread or systematic attacks against the civilian population, which may amount to crimes against humanity, it said.

There was a clear pattern of snipers shooting at demonstrators, and in some cases targeting people trying to evacuate the wounded. In hospitals there were several reports of security forces killing injured victims by putting them alive in refrigerators in hospital morgues.

The United Nations plans to send a team to Syria this weekend to assess the humanitarian situation there, U.N. humanitarian chief Valerie Amos said on Thursday.