US president Barack Obama has called for Syrian president Bashar al-Assad to step down, and issued a new executive order freezing all Syrian assets within American jurisdiction.

American citizens are now also barred from any business dealings with the Syrian government.

“For the sake of the Syrian people, the time has come for President Assad to step aside,”  Obama said in a statement, the first direct call for al-Assad's resignation from the US government.

But Obama also emphasised that Washington "cannot and will not

impose this transition upon Syria" and vowed to heed Syrians' "strong

desire that there not be foreign intervention in their movement."

"It

is time for the Syrian people to determine their own destiny, and we

will continue to stand firmly on their side," he said.

The lethal crackdown Syrian forces have waged against pro-democracy protesters across the country drew censure from a United Nations human rights panel in Geneva this week.

It said government forces may have committed crimes against humanity by carrying out summary executions, torturing prisoners and harming children in their crackdown against opposition protesters.

The panel recommended that the Security Council refer Syria to the International Criminal Court for prosecution of alleged atrocities, an issue expected to be discussed by the council later in the day.