The Al Aqsa Martyrs’ Brigades, a violent offshoot of the Fatah movement of Palestinian President Mahmoud Abbas, said the five were members. The group refused to say what they were doing.
The militant group said the explosion was not caused by an Israeli air strike, after first blaming Israel. The Israeli military said it had no forces inside Gaza and denied involvement in the incident.
Israel often attacks tunnels under the border, saying they are used for smuggling weapons and explosives into Gaza.
Three weeks ago the IDF attacked the home of a Hamas member in Rafah, under which a tunnel was dug to smuggle weapons. After the attack, a number of follow-up explosions were heard, backing the IDF's claim on the existence of many weapons in the area.
The weapons-smuggling tunnels are being dug up as part of ongoing efforts to collect arms ahead of a possible all-out clash between the various factions and a possible conflict with Israel.
Hanan Greenberg contributed to the report