Syrian opposition groups have gathered in the Qatari capital, Doha, for a key meeting on how to form a more united front against President Assad.
The meeting could lead to a replacement body for the Syrian National Council (SNC), the main opposition in exile.
Rebel forces in Syria have criticised the SNC as out of touch, and the opposition is also split ideologically.
The Doha talks come a day after rebels in northern Syria launched an offensive to take control of a key airbase.
The SNC will be looking to broaden its ranks and agree on a common platform at the conference, the BBC's Jim Muir reports from Doha.The SNC will be holding four days of intensive internal meetings aimed at overhauling its structures completely, our correspondent says, bringing in new, young elements closer to events on the ground, and producing a new leadership.
It will hold talks with the Syrian National Initiative, a group of influential and respected opposition figures who are proposing the creation of a unified leadership body that would later produce a government in exile, possibly as early as next month.
Respected dissident Riad Seif is apparently being suggested by the US as the head of the new government in exile.
"An alternative to the regime is dearly needed," Mr Seif told Reuters news agency.
"We are talking about a temporary period that begins with forming a political leadership until a national assembly that represents all Syrians meets in Damascus, once Assad falls," he said.
Mr Seif was among more than 20 opposition leaders who gathered in Jordan on Thursday to hammer out proposals for a new leadership.
The participants there issued a statement to quell fears that they were planning to negotiate with President Bashar al-Assad.
"Assad and his entourage leaving power is a non-negotiable precondition for any dialogue aimed at finding a non-military solution, if that is still possible," the statement said.
The US is hoping the new leadership will help bring a successful conclusion to an uprising that has killed more than 36,000 people since protests against President Assad erupted in March 2011.Earlier this week, American officials signalled the opposition needed to be expanded from just the SNC to take in more of those operating inside Syria.
Divisions have arisen not just between those in Syria and opposition figures abroad, but also between Islamist and secularist groups.
A previous opposition meeting in Cairo in July accepted that the Assad government must fall but failed to appoint a committee to act for the opposition internationally.
Representatives at Doha will include various other religious and secular groupings, plus Kurdish figures and dissident members of Mr Assad's Alawite sect.
Inside Syria, rebels on Saturday attacked the strategically important Taftanaz base in the north with multiple rocket launchers, mortars and other weaponry.
Activists said the fighting at Taftanaz, which is crucial for government supply lines into northern Syria, continued into Saturday evening.
The state-run Sana news agency said on Sunday that government forces had confronted what it called several armed terrorist groups, adding that "the armed forces killed a large number of terrorists and destroyed their vehicles".
Also on Sunday, the UK-based opposition activist group, the Syrian Observatory for Human Rights, reported fierce clashes between government troops and rebels near a security building in Damascus.