GENEVA (Reuters) - The number of Islamist rebels in eastern Aleppo who are not protected by any ceasefire deal, and can therefore be legitimately targeted, is far smaller than an estimate given by the United Nations, diplomatic sources have told Reuters.

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Jabhat Fateh al-Sham (JFS) is one of Syria’s most powerful rebel forces, and under its former name - Nusra Front - it was designated as a terrorist group because of links with al Qaeda.

Russia, Syria and Iran say JFS’s presence in besieged eastern Aleppo justifies their attacks, which began despite a U.S.-Russian ceasefire, but the U.N. has urged them to stop using the group’s presence as an “easy alibi” to bomb the city.

None of them have given estimates of JFS numbers in Aleppo.

Last week U.N. Syria envoy Staffan de Mistura said there were a maximum of 900 JFS members out of a total 8,000 rebel fighters within the besieged opposition-held area.

Several sources independently told Reuters that de Mistura’s figure for JFS fighters was far too high, and the real number was no more than 200, perhaps below 100. One Western diplomat said it possibly had no more than a “symbolic” presence.

“The Russians indicated they wanted to talk about de Mistura’s plan which is to get the rebel fighters out of Aleppo. We are a bit skeptical. De Mistura spoke about 900 fighters from Fateh al-Sham, we estimate (the number) at much less,” one senior French diplomatic source said.

The issue will be discussed on Saturday when foreign ministers from countries including the United States and Russia, which back opposing sides in the war, gather in the Swiss city of Lausanne for talks aimed at halting the spiraling violence.

De Mistura has offered to personally escort the fighters out of the city in return for a ceasefire observed by all parties.

He has already scaled back his estimate.

On Sept. 25, he said that he had information that more than half the fighters in eastern Aleppo were from JFS but gave no total figure. Last week he cited a “much more updated analysis”, and he said the 900 figure was “in my opinion, quite reliable”.

His office declined to comment on where he had got the 900 figure from, but quoted remarks he made to a closed-door session of the U.N. Security Council last week: “If al-Nusra are 900 people, then 900 will have to leave.

If they are less, then less will leave. We will be actually involved in that. The point is that every Nusra fighter will leave the city and the rest will stay if they want to.”

A Jabhat Fateh al-Sham spokesman did not immediately comment on the number.