Syrians are being urged to rally against President Bashar al-Assad on the first Friday of Ramadan after the US turned up the heat on his regime over a crackdown it says has killed more than 2,000.

"God is with us, are you?" Internet-based activists said in a call for people to take to the streets in massive numbers yet again after the main weekly Muslim prayers.

The Assad regime has sought to crush the democracy movement with brutal force, killing more than 1,600 civilians and arresting thousands of dissenters, according to human rights activists.

Its latest crackdown has centred on the flashpoint protest hub of Hama, where at least 30 people were killed on Wednesday when tanks shelled the city centre.

On Thursday night Syrian forces shot dead four protesters near Damascus and in southern Syria following nightly Ramadan prayers when they fired at demonstrations demanding the overthrow of President Bashar al-Assad, activists' organisations said.

Abdullah Abazeid, a member for the Syrian Revolution Coordinating Committees, said three protesters were killed and at least ten wounded in the town of Nawa near Deraa, cradle of the five-month uprising against 41 years of Assad family rule.

"Demonstrations have been breaking out daily after 'tarawih' (prayers following the nightly breaking of fasting during the Islamic month of Ramadan) in Deraa and the surrounding towns," Abazeid told Reuters.

The Local Coordination Committees, another activists' organisation, said another protester was killed in the Damascus suburb of Qadam when four buses full of security police surrounded a demonstration there and fired at the crowd.

That prompted harsh words from Washington and Moscow, with Russia hinting at a possible change of heart after stonewalling firm UN action against Syria, its ally since Soviet times.

The White House said the deadly crackdown has put Syria and the Middle East on a "very dangerous path," as Washington extended a raft of recent sanctions to include a businessman close to Assad and his family.

In toughening its stance, President Barack Obama's administration appeared to be moving toward a first direct call for Assad to go, a step it has so far resisted, following an escalation of violence in the revolt hub of Hama.

Secretary of State Hillary Clinton said the United States would urge the Europeans, Arabs and others to do more to press Syria to stop its deadly crackdown.

Clinton said Assad's regime was responsible for the deaths of more than 2,000 people, repeating that Washington believes the embattled president has "lost his legitimacy to govern."

White House spokesman Jay Carney meanwhile hardened US rhetoric.

"It has become very clear around the world that Assad's actions place Syria and the region on a very dangerous path," he said.

"Assad is on his way out... we all need to be thinking about the day after Assad as Syria's 23 million citizens already are.

"It is very safe to say that Syria will be a much better place without President Assad."

The US Treasury Department froze the US assets of Mohammad Hamsho and his company, Hamsho International Group, and prohibited US entities from engaging in any business dealings with them.

The US government said Hamsho was "one of Syria's top businessmen" with interests in multiple sectors and a "close business associate" of Assad's brother Maher, serving as a "frontman" for him and a number of his businesses.

Maher heads the army's elite Fourth Division, with observers saying he has spearheaded the crackdown.

The call for Friday's Ramadan protests came from activists on Facebook group The Syrian Revolution 2011, a driving force behind the demonstrations calling for greater freedoms since mid-March.

Similar calls for demonstrations in the past month have seen hundreds of thousands of people pour onto streets of major protest centres after Friday prayers.

Activists and analysts have meanwhile dismissed as a ploy a new law allowing the creation of political parties alongside the Baath party, as decreed by Assad on Thursday.

"The regime is not serious about transforming the country from a dominant party into democracy and pluralism. Instead, it is trying to carry out some cosmetic work to improve its image," said prominent human rights lawyer Anwar al-Bunni.

The new law is the latest attempt by Assad to appease protesters after the president issued orders in April lifting five decades of draconian emergency rule and abolishing the feared state security courts.

It came after the UN Security Council condemned the crackdown and said those responsible should be held accountable -- in its first pronouncement on Syria since the protests began.

Unable to agree on a formal resolution, the council settled on a non-binding statement condemning "the widespread violations of human rights and the use of force against civilians by the Syrian authorities."

Western powers had hoped for stronger action but were rebuffed by veto-wielding members Russia and China, who feared doing so would pave the way for another military intervention like the one in Libya.

But Russian President Dmitry Medvedev spoke forcefully about the situation on Thursday and called on Assad to "carry out urgent reforms" warning otherwise "a sad fate awaits him and in the end we will have to take some decisions."

And Kuwait on Friday urged a halt the crackdown, expressing its "extreme pain" and calling for dialogue and a political solution to allow for "true reforms that meet the demands of the Syrian people".