Life in Aleppo will become impossible within weeks under relentless Russian and Syrian regime air strikes, the head of the renowned White Helmets volunteer rescue force said Tuesday.

But the quarter of a million civilians trapped in the besieged rebel-held east of the Syrian city believe they will be massacred if they flee or if the city falls, Raed Saleh told AFP.

And the White Helmets, members of a civil defense group that has won international acclaim for their work digging the wounded from bombed-out buildings, fear they will be rounded up and shot.

"The civilians there would seize any opportunity to escape, to go wherever they could go," Saleh told AFP in Washington, where he is on a frustrating quest for international support.

"But nothing is available to provide safety and protection for those civilians. We are worried that they are facing massacre or the kidnapping or the arrest of many of them."

In the eight days since Bashar al-Assad's regime declared an end to a US and Russian-brokered ceasefire on September 9, Aleppo has been hit by 1,700 air strikes, according to Saleh.

Both Russian and regime warplanes have taken part in waves of attacks, he said, using weapons new to the siege and deadly among the packed and crumbling civilian homes.

These have included 19 strikes with powerful "bunker busters" that leave victims entombed in rubble and almost 200 with cluster munitions and phosphorous bombs.

"We have 1,000 casualties, both dead and wounded," Saleh said.

Saleh's figures are impossible to confirm, but international bodies have condemned the bombardment and UN Secretary-General Ban Ki-moon said it may amount to a war crime.

More than 300,000 people have been killed in a civil war now into its sixth year and millions have fled their homes -- an option not even available in besieged Aleppo.

"I think the civilian facilities will not be able to continue providing services for more than a month," Saleh warned, echoing reports from reporters in the city.

"There will be no water and no electricity, no fuel, hospitals will not be able to keep going. If the situation continues like this, I expect a big genocide."

- 'We stand for the victims' -

The population and the outgunned anti-Assad rebel fighters mingled among them fear arrest or worse at the hands of regime forces and their militia allies when Aleppo falls.

And the 122 White Helmet volunteers, who have received international plaudits for their work pulling the wounded from the rubble, could be targeted.

"The White Helmets are from the people and they are subject to the same conditions as the rest," he said. "I'm sure the regime will do its best to assassinate or arrest them."

The White Helmets organized themselves to provide frontline rescue services in rebel-held areas of Syria, they receive some international donations but insist they are independent.

"The biggest need that we have to respond to is to rescue people hit in the air strikes," Saleh said.

"And the air strikes that target civilian communities are usually in Idlib, eastern Aleppo, northern Homs," he added, citing areas under the control of rebel groups.

"It is a very normal human reaction that the people oppose the party that is bombing them," he said of the populations in the areas where his teams operate.

"So when we respond to rescue people and there is a bomber in the sky and there are casualties on the ground, we're not neutral between those two parties," he explained.

"We stand for the victims and it is our responsibility and our duty to work for the victims."

- Diplomatic support -

Saleh, who is based in northern Syria near Idlib and travels in and out of the country through Turkey, was in Washington after a week lobbying world officials at the UN General Assembly.

His group was warmly received by senior international officials in New York, but in the wake of the failed US-Russian ceasefire, he has begun to despair of outside aid.

"We believe there is no need for more resolutions at the UNSC, no need for more decisions by the UN," he said.

"We need conscience to move the political will of the leaders of the world powers to stop the killing in Syria and make those war criminals accountable for their crimes."