The Iranian Foreign Ministry spokesman has denounced the bomb explosions at two Coptic churches in various cities in northern Egypt, warning of bids to sow discord among followers of different divine religions.
On December 11, 28 Coptic Christians were killed in a suicide bombing at a church in Cairo's Abbassia district.
Coptic Christians make up about 10% of Egypt's population of 91 million.
There was no immediate claim of responsibility and the cause of the blast was not yet known.
Tanta is roughly 60 miles (96 kilometers) north of Cairo.
The first blast happened at St. George church in the Nile Delta town of Tanta, where at least 26 people were killed and 71 others wounded, officials said. Pope Tawadros II, head of the Coptic Church, had been attending Mass inside and was unhurt, state media reported.
Bombings at two Egyptian churches killed more than 35 people as they gathered to mark Palm Sunday, officials said, in one of the deadliest recent attacks on the country's Coptic Christians. Tanta's Principal Governor Ahmad Deif told the Al Arabiya news channel that either a bomb went of or a suicide attackers was involved.
Talking at Nile TV news network, the Governor General Gharbiyah 'Ahmad Nteif confirmed that the explosion occurred in Tanta city in the church.
Over the past few years, Egypt is struggling to combat a terrorist wave that killed hundreds of policemen and soldiers since the military removed former Islamist President Mohamed Morsi in July 2013 and blacklisted his Muslim Brotherhood group as "a terrorist organization".
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In February, Christian families and students fled Egypt's North Sinai province in droves after Islamic State began a spate of targeted killings there.
Copts face regular attacks by Muslim neighbours, who burn their homes and churches in poor rural areas, usually in anger over an inter-faith romance or the construction of church.
"So sad to hear of the terrorist attack in Egypt. U.S. strongly condemns".
El-Sissi said in a statement that Sunday's attacks would only strengthen the resolve of Egyptians against "evil forces".
ISIS has claimed responsibility for the bombings of two Coptic Christian Churches in Egypt on Palm Sunday.
Al-Azhar Grand Imam Ahmed El-Tayyeb stressed Al-Azhar's solidarity with the Egyptian Church in the wake of attacks.
Egypt's Copts are one of the oldest Christian communities in the Middle East.
Egyptian media had previously reported that the church in Tanta had been targeted before, with a bomb defused there in late March.