Ten soldiers were killed and six wounded in an attack on an Ivory Coast military post on the northern border near Burkina Faso on Thursday, the army said.

It was not yet clear who carried out the pre-dawn attack, which was the deadliest in Ivory Coast since gunmen from al Qaeda’s North African branch stormed the beach resort of Grand Bassam in March 2016, killing 19 people.

The head of the armed forces, Lassina Doumbia, said in a statement that 10 soldiers were killed and six wounded. He added that one of the assailants was killed.

Earlier, a senior officer at the army chief of staff’s office said 12 soldiers had been killed and seven wounded and added that two other military gendarmes were reported missing.

The assailants were believed to have come from Burkina Faso, said the officer, speaking to Reuters by phone.

Doumbia said that all troops in the region were on alert and that a search operation was under way to find the assailants.

Ivory Coast and Burkina Faso launched a joint military operation last month to tackle the expanding threat from jihadists linked to al Qaeda and Islamic State in the Sahel region.

A week ago, the US Embassy in Ivory Coast banned its staff from traveling to the country’s northern areas bordering Burkina Faso and Mali because of the risk of attacks.

Groups with links to Islamic State and al Qaeda have sought to widen their influence in West Africa in recent years, regularly carrying out deadly attacks.

The landlocked nations of Mali, Niger and Burkina Faso have been worst hit, in part because their unpoliced desert expanses have allowed fighters to cross borders undetected.