More than 60 people were killed by a twin suicide bomb attack at a camp in northern Nigeria for people displaced by an insurgency of the jihadist Boko Haram group, a military and emergency official said on Wednesday.

The attack took place in Dikwa, some 85 km (50 miles) outside the capital of Borno state, centre of the seven-year insurgency, they said.

It took place on Tuesday, but a breakdown in the telephone system had prevented the information being made public earlier.

The two female suicide bombers sneaked into the  internally displaced persons (IDP) camp and detonated themselves in the middle of it, emergency officials and the military source said.

The chairman of the State Emergency Management Agency, Satomi Ahmad, added that 78 people were injured.

The militant group has recently increased the frequency and deadliness of attacks with three at the end of January. At least 65 people were killed outside Borno state capital Maiduguri on Jan. 31.

Since it lost territory to a government counter-offensive last year, Boko Haram has reverted to hit-and-run attacks on villages and suicide bombings at places of worship or markets.

Boko Haram has only rarely targeted camps housing people displaced by the conflict and Tuesday’s attack was the first one to kill victims in Borno state.

The military said militants made one abortive attempt on a camp on the outskirts of Maiduguri on Jan. 31. Boko Haram hit a Nigerian IDP camp for the first time last September, in the Adamawa state capital of Yola.