Islamic State militant group (ISIS) claimed responsibility for the suicide bombing attack that killed at least 25 people at Cairo's main Coptic Christian Cathedral this weekend.

ISIS vowed to continue its war against "polytheism" in a statement circulated on Tuesday by SITE Intelligence Group, that  monitors jihadist activity online.

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According to the SITE, one of the militant's suicide bombers had detonated an explosive belt inside a "Christian temple" at the cathedral complex.

The bombing struck the female worshiper's side of the small church of St. Paul and St. Peter, adjoined to the Coptic Cathedral in the capital's Abbassiya district. Most of the dead were women, attending a weekly sermon. Another 49 people were wounded and at least six children were among the dead.

This would be the first militant attack on a Christian house of worship in Egypt since 2011, which killed 23 people at the Two Saints Church in the coastal city Alexandria. No group, however, has ever claimed responsibility for it.

Meanwhile, last weekend's suicide bombing is the deadliest attack on civilians claimed by the terror group in Egypt since the shooting down of a Russian passenger jet that killed all 224 people aboard in October 2015.

President Abdel Fattah Al Sisi declared a three-day mourning period on Monday at the state funeral for the victims and promised the perpetrators would be brought to justice.

Sisi named the bomber as 22-year-old Mahmoud Shafik Mohamed Mostafa. The leader also said that four people had been arrested in connection with the attack and two fugitives were being sought.

The Egyptian Interior Ministry said that Mostafa was a supporter of the muslim brotherhood, an islamist political organization that Sisi has banned in Egypt. Mostafa has been arrested in March 2014 for carrying arms during a protest but was later freed on bail after two months. He was also wanted in connection with two other cases, according to Newsweek.

Egypt's military is battling a growing insurgency in the northern Sinai Peninsula, known as Sinai Province, with the ISIS and other extremist groups regularly targeting military and government installations as well as security forces and civilians.

TagsISIS, Egypt, Cathedral Bombing, President Abdel Fattah Al Sisi, Mahmoud Shafik Mohamed Mostafa