With angry criticism pouring on from all sides, even from rival Islamist extremist groups, the Islamic State (ISIS) has been forced to make an "extremely defensive" propaganda effort to justify its slaughter of women and children.

In the latest edition of the terrorist group's Rumiyah magazine—which touted the Easter Sunday church bombings in Egypt that killed at least 45 people—the leader of the ISIS faction in Egypt claimed that killing women and children in indiscriminate terror attacks is allowed in Islam, citing Mohamed's use of catapults to lay siege on a medieval city, The Independent reported.

"With regards to those of the non-combatant women and children from among the belligerent Christians who are killed unintentionally, their blood is waste," the unidentified ISIS leader was quoted as saying.

The terrorist leader also claimed that it is "permissible" in Islam to murder all men and take women, children, the elderly and ill as slaves.

Opponents of ISIS, even rival jihadist organizations, have blasted the terrorist group for its indiscriminate killing of women and children – particularly Muslims.

Jean-Marc Rickli, a research fellow at King's College London and the Geneva Centre for Security Policy, told The Independent that even some ISIS supporters have denounced the terrorist group's attacks on churches in the Arab world, saying they are "crossing a red line."

Muslim critics of ISIS pointed out that Christians living in Muslim lands during the Islamic Golden Age were afforded protection under Sharia as long as they paid a special tax.

However, ISIS in Egypt claimed that "belligerent" Christians in Egypt deserve no protection.

Rickli said ISIS faces "a double challenge": It's losing ground in the Middle East, especially in Syria, while its rival, al-Qaeda is bolstering theirs.

"They still have to maintain momentum to attract new candidates but also they have to justify that what they're doing, which is contrary to what most people would think is right," he said.

Last week, an ISIS leader in Egypt issued a warning to Muslims, telling them to stay away from Christian gatherings and suggesting that it would launch new attacks on "legitimate targets," CP earlier reported.

"We are warning you to stay away from Christian gatherings, as well as the gatherings of the army and the police, and the areas that have political government facilities," the anonymous leader said in an interview with Al Naba, a weekly ISIS publication as quoted by Reuters.

The ISIS warned that its militants will launch more attacks in the days ahead. "The Crusaders and their tails from the apostates must be aware that the bill between us and them is very large and they will be paying it like a river of blood from their sons, if God willing," the unidentified terrorist leader said.