ISIS has seemed able to replenish its fighters as fast as airstrikes and ground forces have been able to kill them.

But the militant group still might be considering forcing military conscription within its territory.

ISIS authorities in Raqqa, Syria, are allegedly forcing males over the age of 14 to register with the militant-run government. That's according to Abu Ibrahim al-Raqqawi, an activist with the group Raqqa is Being Slaughtered Silently, who spoke with Business Insider via Skype.

"People are very afraid that [ISIS] will force … one male from every family to join them to fight," he said. "People think it's part one from a recruiting way, and the part two will be, 'Oh, you need to go to fight, every family will need to give one boy.'

"There are rumors in the city that, 'Boys must not sit at home like women. There is a war. They need to go to jihad.'"

Raqqawi (who uses a pseudonym) still travels back and forth from Raqqa, the capital of ISIS' self-declared caliphate, with the help of smugglers. His family remains there.

Raqqa is Being Slaughtered Silently has been documenting ISIS abuses since the militants swept through Iraq and Syria and claimed territory for its caliphate, which it markets as an Islamic utopia.

ISIS, also known as the Islamic State and ISIL, also makes it very difficult for people to leave its territory, Raqqawi said.

"They are not allowing women to leave the city if they are not over 45 years old, and they are not allowing boys to leave the city if they are not over 19," Raqqawi said. "Leaving the city is really now hard. After the statement that, 'We want to register every boy in the city over 14,' people are very afraid of the recruiting and want to leave."

Other activists have told similar stories of ISIS registering young males in Raqqa.

Hamoud al-Mousa, who is from Raqqa but now lives in Turkey, told Al Jazeera America there was "a fear men of the city will be dragged into this grinding war they have no hand in, with the primary goal of protecting Daesh."

Some experts are skeptical that the registration requirements are a sure sign of military conscription.

"Although it might be for conscription, I doubt it is the intention — since ISIS still allows members to leave if they have a good reason to leave the organization,"Hassan Hassan, an expert on Syria who recently cowrote the book "ISIS: Inside the Army of Terror," told Business Insider.

"Arming the population carries risks for the group and it'll prefer to recruit rather than to hand weapons to youngsters after it worked hard to disarm the population. The move is likely to keep a record of the local population, especially at this age, and to intimidate them."

Still, ISIS pours a lot of resources into building up a powerful fighting force as a US-led coalition considers ramping up attacks on the group. ISIS could be wary of forthcoming coalition attempts to drive the jihadists out of Raqqa, and some experts say estimates of how many fighters ISIS counts among its ranks are overblown.

"The core of ISIS is less than what's being reported," Hassan said. "The rank-and-file is probably less than 50,000."

Hassan explained that the core of ISIS is made up of "true believers" in radical Islam who have joined the group. ISIS also calls on more peripheral people — "allies" and "contractors" who can fight on behalf of ISIS but might not fully subscribe to the group's radical ideology — to do its bidding, providing weapons and money in exchange for fighting.

And ISIS wouldn't rely solely on the possibility of forced military conscription. To grow its ranks of core members, ISIS has been indoctrinating children who live in its territory and offering perks to fighters and their families as an incentive to join the group.

"They target children a lot because it's very easy to recruit them, more so than [adult] guys," Raqqawi said. "Boys it's affecting a lot. They see all the time guns and calls for jihad. So they love to go, sometimes they think it's a game."