The United States for the first time explicitly called on Thursday, August 18, for Syrian President Bashar al-Assad to step down because of his brutal crackdown on five months of protests against his authoritarian rule, Reuters reported.

President Barack Obama also imposed fresh American sanctions to freeze Syrian government assets under U.S. jurisdiction and to bar U.S. individuals or companies from transactions with Assad's government.

The moves are intended to ratchet up pressure on Assad, who has used his military and security forces to attack protesters seeking an end to the 41-year reign by the Syrian president and his late father, Hafez al-Assad.

Analysts said 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 but would likely be followed by European nations with greater economic leverage.

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

"For the sake of the Syrian people, the time has come for President Assad to step aside," he added.