The surge in violence has trapped millions of Syrians, turned sections of Damascus into ghost areas, and sent tens of thousands of refugees fleeing to neighbouring Lebanon.
The U.N. Security Council has approved a 30-day extension for a ceasefire observer mission, but Ban has recommended changing its focus to pursuing prospects for a political solution - effectively accepting there was no truce to monitor.
Diplomats said only half of the 300 unarmed observers would be needed for Ban's suggested plan, and several monitors were seen departing from Damascus on Saturday.
Speaking two days after Russia and China vetoed a resolution to impose further sanctions on Assad's government, Ban called on the Security Council to "redouble efforts to forge a united way forward and exercise its collective responsibility".
"The Syrian government has manifestly failed to protect civilians and the international community has collective responsibility to live up to the U.N. Charter and act on its principles," he said.
Regional and Western powers have voiced concern the conflict might become a full-blown sectarian war that could spill across borders. But Assad's opponents remain outgunned and divided.
French Foreign Minister Laurent Fabius, speaking after contacts with the head of the Arab League and Qatar's prime minister, said all three agreed that it was time for Syria's fractured opposition to prepare to take charge of the country.
"We would like to see the rapid formation of a provisional government representing the diversity of Syrian society," said Fabius. Syria's main political opposition group, the Syrian National Council, operating in exile, has so far failed to unite Assad's disparate foes on a united political platform.
On the military front, a senior Syrian defector said Assad could now rely only on an inner core of loyal army regiments, adding "the collapse of the regime is accelerating like a snowball".
General Mustafa Sheikh said Assad's forces were transporting chemical arms across Syria for possible use against the rebels.
"The regime has started moving its chemical stockpile and redistributing it to prepare for its use," said Sheikh, citing rebel intelligence obtained in recent days.
The White House said on Saturday it was concerned about what might happen to chemical weapons in Syria but believed Damascus's stockpile "remains under government control". www.trust.org