The United States for the first time explicitly called on Thursday for Syrian President Bashar al-Assad to step down and imposed new economic sanctions likely to be followed up by the European Union.

President Barack Obama made his appeal after five months of a brutal Syrian government crackdown against protesters seeking an end to the 41-year reign by the Syrian president and his late father, Hafez al-Assad.

The fresh U.S. sanctions would freeze Syrian government assets under U.S. jurisdiction, bar U.S. individuals or companies from transactions with Assad's government and ban U.S. imports of Syrian petroleum.

The U.S. sanctions are likely to have limited impact because of the low level of U.S.-Syrian trade and the minimal U.S. imports of Syrian oil, analysts said, but would likely be followed by European nations with greater economic leverage.

Britain, France, and Germany also demanded that Assad go, as did the EU, whose foreign policy chief, Catherine Ashton, said the bloc's 27 members were preparing to target more Syrian entities and looking at ways to broaden their sanctions.

EU diplomats are scheduled to discuss sanctions in Brussels on Friday, opening the way for any new measures to go into effect as early as next week.

While the United States has previously called for Assad to embark on democratic reforms or to get out of the way, Obama's statement marks the first unambiguous demand for him to go.

Obama's decision to take a harder stance toward Assad appears to reflect a U.S. conclusion that he is incapable of reform as well as growing condemnation of the Syrian leader, including from influential Arab nations such as Saudi Arabia.

For the sake of the Syrian people, the time has come for President Assad to step aside, Obama said in a statement.

His calls for dialogue and reform have rung hollow while he is imprisoning, torturing, and slaughtering his own people, Obama said in the written statement announcing the sanctions.

Assad told U.N. Secretary-General Ban Ki-moon that military and police operations against pro-democracy protesters have stopped, but activists and residents reported more killing overnight and more shooting on Thursday.

Separately, U.N. human rights investigators said Assad's forces had carried out systematic attacks on civilians, often firing at short range and without warning, killing at least 1,900 civilians, including children.

NO U.S. MILITARY THREAT

A U.S. official all but ruled out military action to force Assad out. Barring that, and with a weak and divided political opposition, it seemed possible the Syrian leader could defy calls for his removal for some time to come.

The United States is trying to push two other Arab leaders from power: Muammar Gaddafi of Libya, where NATO air strikes are helping anti-Gaddafi rebels, and Ali Abdullah Saleh of Yemen, where Washington is relying on diplomacy.

U.S. officials told reporters that they were confident the balance had shifted in Syria, that the Syrian people were no longer afraid of Assad and would no longer tolerate his rule.

Syria is a country that is only now emerging from what in effect has been 40 years of an induced political coma, said one official, speaking on condition of anonymity. We can't predict how long this transition will take. Nothing about it is likely to be easy but we are certain that Assad is on the way out.

The new U.S. sanctions immediately freeze all Syrian government assets subject to U.S. jurisdiction and prohibit any transactions involving the Assad government.

They also ban U.S. imports of Syrian petroleum or petroleum products -- a tiny fraction of U.S. oil imports -- and prohibit U.S. citizens from operating or investing in Syria.

The United States has already imposed several rounds of sanctions on Assad and other Syrian officials and financial institutions but these are believed to have had little impact given the limited nature of U.S.-Syrian dealings.

According to U.S. government figures, in 2010 the United States imported an average of just 9,000 barrels per day of oil from Syria, a tiny fraction of the U.S. average consumption of 19.18 million barrels per day.

What was announced today will have a very limited impact as it doesn't really prevent anyone from outside the U.S. dealing with Syrian oil, said Greg Priddy, global oil analyst at Eurasia Group in Washington.

With 95 percent of Syrian oil exports going to the EU, you might see more action there, he added.

Oil sales account for a third of Assad regime revenue, so banning their purchases, with possible measures by the EU tomorrow, would strike a crippling blow to the Assad regime, said Andrew Tabler, a fellow at the Washington Institute for Near East Policy, a Washington-based think tank.

(Additional reporting by Deborah Charles,Andrew Quinn, Ayesha Rascoe, and Warren Strobel in Washington; by David Sheppard in New York and by Justyna Pawlak in Brussels; Editing by Will Dunham.)