Unknown assailants detonated a bomb outside a Christian school in Gaza City before dawn Friday, causing no injuries.

The explosion was heard in surrounding neighborhoods at around 4 a.m. Damage was visible at the entrance to the Zahwa Rosary School, which is run by Catholic nuns but caters mainly to Muslim students.

Two nuns were in their convent adjacent to the school when the bomb went off, a school official said, and were shaken but unharmed. The official declined to be named, saying she was frightened by the incident and concerned for her safety.

The incident appeared to be the work of a poorly trained individual or group, she said - police told school officials that the bomb had been set incorrectly, and it caused little damage.

The bombing was the latest in a string of attacks on Christian institutions in the overwhelmingly Muslim territory. In the most serious attack, a local Christian activist was murdered in October. His killers have not been found.

Friday's bombing was not the first attack on the school run by the Rosary Sisters. The school was ransacked in June, 2007, along with the nuns' adjacent convent, during a week of intense fighting that ended with Hamas' seizure of power...