He says that more than 60 wounded people were taken to two government hospitals. Islamic State later claimed responsibility for the attack. It provided no further details.
IS claimed the attacks via its Aamaq news agency, after recently warning that it would step up violence against Egypt's Christians.
Analysts say Sunday's bombings suggest the group is lashing out as it finds itself under increasing pressure in other countries.
Steffen Seibert told reporters that it was important to preserve the peaceful coexistence of Muslims and Coptic Christians in Egypt.
Israel and the Palestinian Hamas movement both condemned the bombings too.
IS named the two attackers, whose noms de guerre suggest they were Egyptians.
"The attack.will only harden the determination (of the Egyptian people) to move forward on their trajectory to realise security, stability and comprehensive development", Sisi said in a statement.
According to a statement from the Ministry of Human Capacities, two Coptic women who were injured during the attack are now receiving treatment in Hungary. Dozens have been killed in sectarian clashes.
Hamas took over Gaza in 2007 after ousting forces loyal to Palestinian President Mahmoud Abbas.
Terrorists like ISIS accuse Copts of supporting the military overthrow of Islamist president Mohamed Morsi in 2013, which ushered in a deadly crackdown on his supporters.
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Hamas has been attempting to improve ties with Cairo.
Tanta is roughly 60 miles (96 kilometers) north of Cairo, in the Nile delta.
In Washington last week, Trump praised hard-line Egyptian leader Sisi for doing a "fantastic job" and solicited his help in the fight against terrorism and violent extremists.
Funerals were also held Monday afternoon in Alexandria for two policewomen killed in the suicide attack. Assiut has a significant Christian population.
"We pray for our Coptic Orthodox sisters and brothers who continue to be resilient in the face of ongoing and escalating attacks", His Grace Bishop Angaelos, General Bishop of the Coptic Orthodox Church in the United Kingdom said in a statement released after the attack.
In a separate statement, the Security Council called the bombings "heinous and cowardly", and reaffirmed that terrorism was one of the most serious threats to global peace and security.
"We also convey our deepest condolences to the victims' families while wishing the injured a speedy recovery", the statement said. Francis learned of the attack during Palm Sunday observances in St. Peter's Square. Hours later, a suicide bomber killed at least 11 people and injured 25 in front of a church in Alexandria, according to reports.
No one immediately claimed the attack. The Interior Ministry has confirmed he was not harmed.
The Tanta bombing was also caught on camera during a live televised stream of the church service.
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