UN Secretary General Ban Ki-moon condemned what he called the “deplorable” attack on Tunisia’s national museum that left at least 17 tourists, including a South African, dead on Wednesday. The UN Security Council also slammed what it called “this heinous act.”

Tunisia said it would deploy the army to major cities and arrested four people on Thursday after militant gunmen killed 20 foreign tourists visiting the national Bardo museum, the worst attack on the north African country in more than a decade.

Nine people have been arrested in connection with Wednesday’s gun attack in Tunis that left 23 people dead, including 20 foreign tourists, the Tunisian presidency says. It alleged that four were directly linked to the attack and five had “ties to the cell”.