BAUCHI, Nigeria (Reuters) - Gunmen have killed more than 30 gold miners in a remote area of northern Nigeria, police said on Tuesday.

The attackers raided a camp of artisan miners late on Monday in the Maru area in the northwestern state of Zamfara, police spokesman Muhammed Shehu said.

The gunmen stole nothing, he said, adding that the motive for the attack was unclear.

In March gunmen attacked a camp in the same state, killing one miner in what a police official described as possible conflict between miners.

Nigeria has been trying to attract investment in its nascent mining sector but insecurity and a lack of data has dampened appetite of foreign firms.