Experts tracking the "Islamic State of Iraq and Syria" (ISIS) cannot agree on how many active members the group still has in Syria and Iraq, AFP reported Thursday (August 23rd).

This is partly because much of its former territory remains an inaccessible warzone, they said.

The Soufan Group, a security consultancy, noted that in December the international coalition had estimated the number of ISIS fighters across Iraq and Syria at 1,000.

Last week, the US Defence Department estimated there could be up to 17,000 left in Iraq and 14,000 in Syria.

UN observers gave yet another estimate in a recent report -- somewhere between 20,000 and 30,000 between Iraq and Syria.

"Today, the main pitfall in reaching a trustworthy figure is that we do not know how many among their ranks have been killed in [various] operations," said Jean-Charles Brisard, head of the Centre for the Analysis of Terrorism.

The bodies of ISIS elements killed in air raids conducted by various parties are in many cases still under the rubble, he said, "so, until we know for sure that they have been killed, they are considered to be still alive".

Tore Hamming, an expert on extremism from the European University Institute, said the numbers are "impossible to assess".

"First you need to define what an ISIS fighter or member is," he said. "The new [UN] number, does that only cover people with a gun, or everybody working for the cause of ISIS? Nobody knows."

The Soufan Group said ISIS had "slipped from a proto-state back to an insurgent group", but added that it remains a threat.