Women walk in front of a huge banner of Egypt's President Abdel Fattah al-Sisi in downtown Cairo, March 13, 2014.

Cairo - A military official says the death toll among troops from a brazen attack on a border post in Egypt's western desert has risen to 22.

The official said one of the wounded troops died Sunday.

The attack was the deadliest suffered by the military in recent history.

Officials, including President Abdel-Fattah el-Sissi, a former military chief, called it a terrorist attack that won't go unpunished.

El-Sissi declared a three-day mourning period.

The official spoke on condition of anonymity because he was not authorised to speak to the media.

Gunmen armed with rocket-propelled grenades attacked the border post in the country's largest province al-Wadi al-Gedid, which straddles the Libyan and Sudanese borders, before sundown Saturday, causing an explosion in the ammunition warehouse.

Three attackers were killed in brief clashes. - Sapa-AP