Damascus and its Russian ally have used the presence of fighters with former Al-Qaeda affiliate Fateh al-Sham Front -- previously called Al-Nusra Front -- to justify their offensive on rebel-held eastern Aleppo.
The top UN envoy for Syria, Staffan de Mistura, has also called on the fighters to leave the city to help enable aid to reach the estimated 250,000 civilians living under siege.
But Khaled Khoja, a top negotiator with the High Negotiations Committee (HNC) opposition umbrella group, insisted that trapped and desperate residents and the armed rebels trying to defend them had been left with no option but to accept help from the Islamist fighters.
"The inaction of the international community in terms of breaking the siege of Aleppo allowed al-Nusra to intervene in this battle," he told AFP in Geneva late Monday.
"You cannot ask the people who are suffering inside Aleppo because of the siege to refuse the help from anyone," he said.
His comments came after opposition factions allied with jihadists launched a major assault on Friday, backed by car bombs and salvos of rockets, to end the regime's three-month encirclement of the city's eastern districts.
He said there were only around 300 Nusra fighters in eastern Aleppo out of around 20,000 rebels -- numbers that differ wildly from a UN estimate of around 8,000 rebels in the besieged part of the city, including some 900 who belong to Fateh al-Sham.
- Not targeting civilians -
Khoja rejected widespread criticism of opposition fire on civilians in western Aleppo, stressing that "as HNC we don't accept civilian targets".
According to the Syrian Observatory for Human Rights, a Britain-based monitor, heavy rebel rocket fire since Friday has killed 51 civilians, including 18 children.
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Khoja acknowledged that there had been civilian casualties as the rebels struggle to break the siege, but blamed international backers like the United States who had refused to provide accurate weapons.
He said the rebels "are not targeting civilians, they are targeting the regime, but... the bombs they are using are not perfect bombs".
On the other side, he said, "the Russians are targeting civilians, the regime is targeting civilians. It's uncomparable."
Khoja also voiced hope that the US election next week would push Washington to more actively support the opposition in Syria.
"We heard (Democratic presidential candidate Hillary) Clinton talking about civilian protection, no-fly-zones, which is something we welcome and that we demand," he said.
He was more wary of a possible win by Republican candidate Donald Trump, who he said came across as "very close to (Syrian President Bashar al-) Assad and to the Russians."
But he said that once Trump received more information about the Syrian conflict "we hope he can change his attitude".
Developments in Washington aside, Khoja voiced little optimism that the UN-backed peace talks could resume soon.
"We can't talk about a political process unless we alleviate the suffering of the people on the ground. They are suffering from sieges, from starvation, from bombing and shellings," he said.
But in a small sign of progress, he and other HNC negotiators were back in Geneva for the first time since they walked out of talks six months ago amid soaring violence on the ground.
Khoja said they were in the Swiss city not for political talks but for technical discussions with de Mistura's staff on the issue of detainees.
Describing Monday's seven-hour meeting as "fruitful", he said the negotiators had agreed to return at a later date for more discussions on securing the release of the more than 200,000 people they estimate are being held by the regime and its allies.