The group posted a picture of the child on the messaging app "Telegram" with the words: "Even the Royal Family will not be left alone."

According to Fox News, the militants also shared Prince George's school address.

The message also reportedly included words from a jihadi song which translates to "When war comes with the melody of bullets, we descend on disbelief, desiring retaliation."

This isn't the first time ISIS has used Telegram to recruit terrorists and threaten people.

Barry Spielman, whose internet surveillance company Sixgill monitors Telegram, says the threats need to be "taken seriously."

"This threat to Prince George is chilling," he told The Star. "These are very worrying times and it is critical we try to uncover these plots. Telegram is where many of them are being hatched as we speak."

ISIS allegedly used the app to recruit the terrorists responsible for Berlin's Christmas market attack.

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