The US State Department has urged Americans in Syria to leave immediately and advised those who remained to restrict their movements.

The warning came as the Syrian government intensified a violent crackdown on opposition protesters.

Meanwhile US congressional calls grew for Barack Obama’s administration to impose severe new sanctions on President Bashar Assad’s regime.

In the warning, the State Department said Americans should leave Syria while commercial flights and other transport were still available “given the ongoing uncertainty and volatility of the situation”.

It noted that Syrian authorities had imposed tight restrictions on the ability of US and other diplomats to move around the country.

The advisory echoed a warning issued in late April when the department ordered the non-essential personnel and the families of all American staff at the US embassy in Damascus to leave the country. But it came as the Assad regime has stepped up efforts to quell the uprising with military force, particularly in the opposition stronghold of Hama where as many as 250 people have been killed since last Sunday.

The crackdown has drawn widespread international condemnation and the Obama administration has said that Assad has lost his legitimacy and that Syria would be a better place without him in power.

Last Thursday, US secretary of state Hillary Clinton said the US believed that the regime had killed more than 2,000 people in trying to put down the unrest that is now in its fifth month.

Congress, meanwhile, urged President Obama to impose tougher sanctions on the Assad regime. Last Thursday the Treasury Department added an Assad confidante and his firm to a US terrorism blacklist, which already includes Assad and some of his inner circle.

In a letter to Mr Obama sent last Friday, 221 members of the House of Representatives called on the administration to bar all US businesses from operating in Syria and to block all property transactions with the Syrian government that could be subject to US jurisdiction.

“The threat posed by the Assad regime to the US, to our allies, and, most of all, to the Syrian people is stark and growing,” wrote the chairman and ranking member of the House Foreign Affairs Committee, Ileana Ros-Lehtinen and Howard Berman.