Militants attacked a military checkpoint in Al-Wadi Al-Gedid governorate in southern Egypt on Saturday, killing 21 border guards, announced army spokesman Mohamed Samir.

According to Samir, the slain were killed during an exchange of fire with the unknown gunmen which lead "to the exploding of the checkpoint's ammunition storage" which he says was fired at using rocket-propelled grenades.

Another four soldiers were injured "in addition to the killing of some terrorists" said Samir.

The army's statement added that two vehicles carrying ammunition and primed to explode, were confiscated at the scene and defused.

An anonymous security source quoted by MENA had originally described the assailants as smugglers.

According to Al-Ahram Arabic news website, the same checkpoint was targeted two months ago when five army conscripts and an officer were killed by gunmen.

Al-Wadi Al-Gedid is a large governorate that borders both Libya and Sudan. According to MENA, the attacks took place at a checkpoint at the 100km mark on the road from Cairo to Farfara, a local town.

The attack was reportedly carried at iftar, the post-sunset meal when Muslims break their fast during Ramadan, resembling at attack in August 2012 in which 16 Egyptian soldiers were killed by militants in Sinai, close to the border with Israel, during Ramadan at the time of iftar.

Saturday's attack is the biggest such single act of violence against army or police since last August when 25 police conscripts were killed near Sinai's Rafah.

Militant attacks targeting police and military have been commonplace since the ouster of president Mohamed Morsi last summer amid mass protests against his year-long rule.