Syria's main opposition said Thursday that the fall of the city of Homs to regime forces could scupper any hope of a political solution to the civil war.

The fall of rebel strongholds in Homs, a symbol of the revolt against President Bashar al-Assad's rule, would make any talks with the regime unpalatable to too many Syrians, spokesman Khaled Saleh said.

"Homs could pose a risk to any political solution," he said at a meeting of the Syrian National Coalition in Istanbul held to appoint a new leadership.

"If Homs falls, it will be very difficult for us to explain to the families of tens of thousands of dead Syrians why we negotiated with a regime that shows us day after day that it doesn't want a political solution and that it only wants to kill more Syrians."

The central city of Homs has paid an enormous human and material price for the uprising against Assad's regime that began in March 2011 and has since evolved into fully-fledged civil war.

Regime warplanes were bombing Homs Thursday as part of an assault by loyalist forces to recapture rebel-held neighbourhoods in Syria's third largest city.

Saleh's remarks will throw into further doubt a peace conference hosted by the United States and Russia dubbed "Geneva 2" due to take place later this year.

On Thursday, the coalition also began the process of selecting a new leader from five official candidates who include interim president Georges Sabra and the former president of the Syrian National Council (SNC) Burhan Ghalioun.

The liberal faction headed by veteran dissident Michel Kilo put forward Ahmad Assi Jarba as a candidate, and the secretary general of the coalition Mustafa al-Sabbagh and a spokesman, Louay Safi, will also stand.

Saleh emphasised the new leader would have to heal rifts in the fractured coalition.

"We are at a critical juncture in the revolution," he said. "The coalition knows it is important to respond to the challenges with which we are confronted."

Under pressure from Saudi Arabia, Qatar and other regional powers, the opposition agreed at a previous meeting to accept new groups under the umbrella of the coalition, lessening the influence of a strong Muslim Brotherhood faction backed by Doha.

Syria's main opposition has faltered since the departure of Ahmad Moaz al-Khatib in May in protest at the world's "inaction" over Syria's civil war.