CAIRO, Dec 11 (Aswat Masriya) - Egypt's Orthodox church stressed on Sunday the importance of preserving national unity following an explosion in the vicinity of the Coptic Orthodox Cathedral.

At least 25 people were killed and tens were injured in an explosion that hit St Peter's church building, which is attached to St Mark's cathedral.

A statement by the church said that they "pray for the martyrs and injured." 

"We also pray for their assailants in order to return to their conscience," the statement added. 

Egypt's President Abdel Fattah al-Sisi has declared three days mourning starting Sunday. Prosecutor General Nabil Sadek has ordered an investigation into the explosion.

This comes two days after six security personnel were killed and three others were injured in an explosion in Haram district in Giza. Recently-emerged movement calling itself "Hasam" claimed responsibility for the explosion.

Militants have stepped up attacks targeting security forces in Egypt, since the ouster of Islamist President Mohamed Mursi in July 2013 after mass protests against his rule.