As Syrian President Bashar al-Assad wages a blood-drenched campaign for survival, Washington is stepping up plans for the day after he falls, colored by costly lessons of Iraq's implosion in 2003.

Senior US officials do not know how long Assad can cling on amid an opposition onslaught, but insist his departure is inevitable and say prudence dictates preparing for the inevitable foreign policy headache.

Washington has long called for a "transition" in Damascus, but the defection of Syrian prime minister Riad Hijab this week bolstered growing hopes here that Assad's regime may be crumbling.

"We can begin talking about and planning more what happens next, the day after the regime does fall," Secretary of State Hillary Clinton said in South Africa on Tuesday. I know it is going to happen."

Visions of a post-Assad Syria are so far characterized more by tough questions than answers. There are fears of a collapse of the Syrian state, a sectarian bloodbath, regional security shockwaves and a humanitarian crisis.

The thought that such drama could unfold at the climax of President Barack Obama's re-election fight against Mitt Romney gives the White House another reason to sketch worst case scenarios and possible solutions.

One lesson of the Arab Spring however is that historic forces that have been unleashed often defy efforts by plodding western governments to keep up, leaving officials struggling to frame new policy on the run.

Clinton and other senior officials have repeatedly stressed the need for Syrian state institutions to stay intact, to keep the country together once Assad's political operatives have been swept away.

Officials say that US deliberations are partly informed by the lessons of Iraq, when a US-led deBaathification drive purged officials from Saddam Hussein's party, which helped incubate conditions for chaos and insurgency.

"It's fair to say that precedent is useful to look at," White House spokesman Jay Carney said, but noted other blueprints in Arab Spring nations and that not all regional revolutions fit the same template.

Reva Bhalla, of geopolitical forecasting organization Stratfor, agreed that Washington's painful experience in Iraq was giving US officials pause as they game out Syria's future.

"The US is very conscious of the consequences of its deBaathification policy in Iraq," she said.

"The US would prefer not to see sudden regime collapse in Syria that would potentially force (it) into a military engagement in trying to secure chemical weapons stockpiles before they fall into the hands of jihadists and Iranian-backed non-state actors."

If the Assad regime crumbles, officials say there will be a need to maintain basic services such as water and electricity, requiring technocrats not affiliated with ruthless political cronies of the Assad regime.

Unlike Iraq, planning for Syria's future will not be a largely US production: Washington is already in dialogue with key allies Turkey and Jordan as well as other regional powers keen to crimp Iran's strategic influence.

Obama has repeatedly discussed the Syrian crisis with Turkish Prime Minister Recep Tayyip Erdogan and America's European allies.

Clinton meanwhile is due back in Turkey at the weekend.

Last week, Secretary of Defense Leon Panetta was in Jordan to discuss Syria after Assad with King Abdullah II. And Washington's former ambassador to Damascus Robert Ford has met Syrian opposition figures in Cairo.

While Washington is standing by its red line of not arming rebels in Syria, there are clear signs of a more robust US role.

Reports last week said Obama had signed a document authorizing the CIA to conduct covert activity in support of Syrian rebels.

He also provided an extra $12 million in lifesaving humanitarian aid to Syrian civilian victims of atrocities.

"This is a grave humanitarian situation, we're talking about half a million people going hungry. We're talking about tens of thousands of people that are flowing over the borders," a senior US official said.

Washington has already provided $25 million in communications supplies and medical equipment for the rebels.

Foreign players in Syria's future face a myriad of problems, ranging from tracking chemical weapons stocks to the capacity of a fractured and makeshift opposition and exiles, and how to move toward political nation building.

There is also a threat that Assad's fall could rip the lid off the country's sectarian and ethnic mix, which could culminate in violence and create a power vacuum that could be exploited by groups such as Al-Qaeda.

Such an eruption, perhaps fanned by Iran, could spread instability throughout a volatile region, potentially threatening Israel, Jordan and reignite sectarian strife in Lebanon.

King Abdullah raised new alarm that Assad could enact "Plan B" and retreat to an Alawite enclave inside a fractured Syria.

"That would be, I think for us, the worst case scenario -- because that means then the break-up of Greater Syria," Abdullah said.