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"Let the crusaders and apostates know that they will pay a huge bill with their son's blood," read an online statement carried by the group's semi-official Amaq news agency.
The statement said the two attacks on the churches, in the cities of Tanta and Alexandria, were carried out by two Egyptians wearing suicide vests.
DPA could not independently verify the authenticity of the statement, but it appeared consistent with previous claims by the group.
The attacks were the latest in a series of assaults on Egypt's Christian minority, which makes up around 10 percent of the population of 92 million and has been repeatedly targeted by Islamic extremists. It comes just weeks before Pope Francis is due to visit Egypt.
Following the attacks, the pontiff expressed his "deep condolences" to the Coptic patriarch, Tawadros II, calling him "my brother," to the Coptic church and "all of the dear Egyptian nation," and said he was praying for the dead and injured in the attack that occurred just hours earlier as Francis himself was marking Palm Sunday in St. Peter's Square.
The pontiff asked God "to convert the hearts of those who spread terror, violence and death, and also the hearts of those who make, and traffic in, weapons."
The attacks met from condemnation around the world, with U.S. President Donald Trump tweeting: "So sad to hear of the terrorist attack in Egypt. U.S. strongly condemns. I have great confidence that President Al Sisi will handle situation properly."
Prime Minister Benjamin Netanyahu's office released a statement in wake of the attacks, saying that, “Israel sends condolences to the families of those murdered in the attack in Egypt and wishes for recovery for the injured. The world must unite and fight terrorism everywhere.”
Egyptian President Abdel Fattah al-Sissi said in a statement he had ordered the immediate deployment of soldiers to assist police in protecting "vital facilities" across the country.
He condemned the attacks and summoned the National Defense Council to an urgent session. According to local reports, he also announced three days of national mourning.
"The attack ... will only harden the determination (of the Egyptian people) to move forward on their trajectory to realize security, stability and comprehensive development," Sissi said.