Abdoulaye Seydou, head of an activist group called M62, appeared before the Niamey criminal court on Monday, public prosecutor Chaibou Moussa said on television in the evening.

He was placed in preventive detention, Moussa said.

M62 and other civic groups have accused the defence and security forces of massacring civilians when they launched helicopter raids on an illegal gold mine last October, on the grounds that the alleged killers of two police officers had holed up there.

The government has acknowledged air strikes were carried out after two police were killed at Tamou, near the border with Burkina Faso.

It said seven people were killed and 24 wounded in the raids but the political opposition and civic groups say the death toll was much higher.

Moussa said that after the "terrorist" attack on the police station and the army riposte, "unidentified individuals" had gone to the gold mine "to burn shops and warehouses used as dwellings".

Citing a police investigation, Moussa said the fires had been set to create fake evidence to incriminate the army and "back up the claims that there had been massacres".

Senior M62 official Falmata Taya acknowledged members of the movement had gone to Tamou two days after the air raids but only "to check out facts on the ground".

Niger, the poorest state in the world according to the UN's Human Development Index, is struggling with jihadist insurgencies in the southwest and southeast of the country.