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ISIS is ordering families with more than one son to give one up to be conscripted into the militant group, a local resident and a lawmaker told NBC News.
The Islamists issued the decree in the Iraqi city of Mosul, which was seized in June.
Ahmed, a 31-year-old resident who asked not to be identified further for his own safety, said ISIS has been broadcasting the message through mosque loudspeakers during Friday prayers and at various "media points" dotted around the city.
"They said it is a sacred obligation to defend Islam against the infidels," Ahmed told NBC News by telephone. He added that some young people were likely to have been "brainwashed and will join them."
ISIS took control of Mosul during an offensive in which its fighters overran large parts of Iraq and Syria. Mosul and the Syrian city of Raqqa represent the main strongholds in the militants' self-proclaimed Islamic State.
The U.S. set up a broad coalition that has carried out more than 1,000 airstrikes in Iraq and hundreds more in Syria since August.
ISIS' ranks within the city have become stretched after sustaining casualties in Diyala and Saladin provinces, according to Mosul lawmaker Nayef al-Shimary, who currently lives in Baghdad because of ISIS' occupation of his city.
"ISIS fighters are forcing people to join their groups because of their great losses," he told NBC News. "They need more fighters and criminals to continue committing crimes against the Iraqis and humanity."
Ahmed, the Mosul resident, added that it was evident the "number of ISIS fighters inside the city is decreasing."
He added: "They try to convince us that we have to defend our city … in order not to live under the same tyranny of the Shiite government, which used to govern us before."
In an exclusive interview with NBC News last month, Iraq’s vice president detailed for the first time a network of anti-ISIS resistance fighters that had carried out hundreds of attacks within the city.
Al-Shimary said there was an "an overwhelming popular rejection" of ISIS from the people of Mosul.
"There is a great resentment because of the crimes committed by those terrorists on the people in Mosul and Iraq in general," he said. "The people of Mosul are waiting for their salvation from the tyranny of those criminals."