Cairo - Egyptian forces killed 21 extremists, at least 19 of them in an airstrike, the military said on Thursday, including three senior figures of the Islamic State group's affiliate in the restive Sinai Peninsula.The announcement came in two separate statements, with the military saying in the first that its air force killed 19 militants and destroyed four vehicles in northern and central Sinai. The statement was coupled with a video that showed its air force striking the vehicles and "terrorist strongholds."The second statement said two additional militants were killed and three injured in central Sinai. It also said security personnel seized two vehicles carrying large quantities of materials used to manufacture explosives. Both were passing through a tunnel that runs under the Suez Canal, according to the statement.On its official Facebook page, the military published photos showcasing stacks of money and three men who had been arrested with the vehicles with their hands cuffed to their backs.According to the military, one of the senior figures killed was responsible for conducting interrogations within the ISIS affiliate and another was in charge of a committee on religious affairs. The role of the third senior figure killed was not immediately clear.The armed forces announcements come nearly two days after militants attacked a checkpoint near the famed St. Catherine's Monastery in south Sinai. The attack was claimed by ISIS.Egyptian police said on Wednesday they killed one of the militants involved in the attack, adding that an investigation was underway to identify the slain militant.The military claims to kill hundreds of militants in the area each year, although it rarely offers proof and journalists and non-residents are banned from the area. Hundreds of security forces have been killed.The ISIS-led insurgency is centered in northern Sinai, but militants have occasionally struck farther south.