By Tom Miles

GENEVA (Reuters) - Russian air cover and Iranian-backed militias are enabling Syria's government to keep fighting in Aleppo, distracting the world from the planned "cleansing" of besieged areas across Syria, an opposition negotiator told Reuters on Thursday.

Basma Kodmani, a member of the Saudi-backed opposition High Negotiations Committee, said forces loyal to President Bashar al-Assad had changed tactics after failing to starve rebels out of besieged zones.

"The threat that we’ve heard over four years – 'starve or surrender' – is now 'surrender or we will wipe you out'. We have really gone a step further in the cynicism and the criminal behaviour of the regime," Kodmani said.

Last week the besieged rebel-held town of Daraya surrendered and the U.N. has said the Syrian army and its allied militia are pressing to repeat that in other besieged opposition zones such as the al-Waer district of Homs, where there are 75,000 people.

"We really need to have an alarm sounded clearly, including by the United States, on those other neighbourhoods," Kodmani said.

"Daraya has become the symbol of the worst that can happen, which can lead to mass displacement of people, ethnic-political cleansing, whatever we want to call it."

In the divided northern city of Aleppo, the U.N. has been trying to broker a regular 48-hour truce to let aid in.

While the U.N. talks grind on, Kodmani said reinforcements of Iraqi militias and an elite Hezbollah unit have arrived in southwestern Aleppo, readying for a big offensive against the rebels and backed by air cover.

"Our fear of course is that Russia will continue to provide air cover," she said. "If Russia was to agree not to lead this battle then it wouldn’t happen. The regime doesn’t have the capacity to provide the air cover any more. This is a war that can only be waged with Russia."

Russia is refusing to apply the 48-hour truce to the whole city, leading the opposition to suspect that it wants to keep bombing rebel-held areas even as aid is trucked into Aleppo down a road held by the army and allied forces, Kodmani said.

"The way you break the circle is to get the Russians to stop: with no air cover, they cannot operate on the ground without it."

(Reporting by Tom Miles; editing by Dominic Evans)