The most recent saw the building illuminated with the UK's Union Jack after the Manchester Arena bombing, which took at least 22 lives and left 59 injured during what was the deadliest attack on United Kingdom soil in 12 years.
Masked gunmen riding in three SUVs opened fire Friday on a packed bus taking Coptic Christians on a visit to a remote monastery, killing at least 28 people, including two little children, authorities said.
Egypt has seen a sharp rise in attacks against Christians, who represent 10 percent of the country's 91 million residents.
On Friday, Egyptian air force had carried out six air strikes on extremist camps in Libya which Cairo says have been training militants who killed 29 Coptic Christians and wounded 24 in southern Egypt.
Egypt's air force struck Derna in February 2015 and March 2016, killing women and children.
Sisi, who has presented himself as a bulwark against Islamist militants in the region, said Egypt would not hesitate to carry out additional strikes inside and outside the country to quash future threats.
A security guard gestures near the site of an attack that killed at least 26 people in Minya, Egypt May 26, 2017.
Isis later claimed responsibility for the Minya bus attack, while the Mujahideen Shura Council, an al-Qaida affiliate that controls the area of Derna targeted by Egypt, released a statement saying it was not responsible for Friday's attack, as it was not its policy to target civilians.
Sisi hads declared a state of emergency in Egypt after the Islamic State bombedf two Coptic churches in April. The gunmen then fired at the vehicles with automatic weapons before fleeing in three 4x4 vehicles, they added.
"They told the men to disembark from the bus, took their identity cards and the gold they had on them, and asked them to the recite the Muslim profession of faith", said Maher Tawfik, whose niece survived.
In past attacks, Egypt had usually identified local jihadists as the perpetrators.
Trump said that the United States "makes clear to its friends, allies, and partners that the treasured and historic Christian Communities of the Middle East must be defended and protected".
During the trip, Francis paid tribute to the victims of the December bombing at Cairo's St. Peter's church, located in close proximity to the St. Mark's cathedral, the seat of the Coptic Orthodox Church.