Four Tunisian police were killed on Tuesday in what the government labelled a “terrorist attack” in a region known as the base for the country’s most dangerous Islamist group.

“On February 18, following a terrorist attack in Boulaaba in the governorate of Kasserine, four heroes of the National Guard were martyred,” the interior ministry said in statement, without giving further details.

Boulaaba is part of a mountain range on the Algerian border which is home to Tunisia’s main jihadist group, the Al-Qaeda linked Phalange Okba Ibn Nafaa.

The group is blamed for an attack which killed 15 Tunisian soldiers in July while dozens of soldiers and police have been killed or wounded in ambushes by the Islamist fighters and landmines.

Tunisia’s armed forces have carried out ground and air campaigns against the group since 2012 without success.

The north African country has seen an uptick in attacks by armed Islamist groups since the overthrow of longtime dictator Zine El Abidine Ben Ali in 2011.