Syrian opposition groups are preparing for a crucial meeting on Sunday in the Qatari capital, Doha, to discuss how to form a more united front.
The meeting could lead to a replacement for the Syrian National Council (SNC), the main opposition body in exile.
The SNC has been criticised as out of touch by rebel forces in Syria, and the opposition is also split ideologically.
The Doha talks come a day after rebels in northern Syria launched an offensive to try win control of an key airbase.
Video posted on the internet on Saturday showed fighters attacking 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, although state media said government forces had repelled the assault.
In recent months, the government forces have been making increasing use of air power to strike areas held by the rebels, who lack anti-aircraft weapons to deter the attacks.
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.
However, other groups and opposition figures will also be present, including respected dissident Riad Seif.
Mr Seif is being apparently being suggested by the US as the head of a new government-in-exile which would be dubbed the Syrian National Initiative.
"An alternative to the regime is dearly needed," Mr Seif told the 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 added.
What may emerge from the meeting is a new leadership body for the Syrian opposition separate from the SNC but including some figures from it, our correspondent reports.
The US is hoping a new leadership will help unify the disparate opposition elements and bring a successful conclusion to an uprising that has killed more than 36,000 people since protests against President Bashar al-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.
'War crime' evidence
The question of apparent brutality by some rebel units has also come to the fore ahead of the Doha meeting.
Video footage which emerged on Friday appeared to show rebel forces beating and then shooting dead a group of prisoners from pro-government forces who were cowering on the floor.
Although nothing certain has been established about the footage, a radical Islamist faction called the al-Nusra Front has been named in connection with the killings at Saraqeb, near the city of Idlib.
The UN has said the video could be evidence of a war crime.
On Friday, the US said it "condemned human rights violations by any party in Syria".
State department spokeswoman Victoria Nuland said: "There is no justification for that kind of behaviour ever. Anyone committing atrocities should be held to account."