Another of those currently being held in custody, Lieutenant Mahamadou Hambaly, was arrested on suspicion of preparing a coup against Issoufou just months after his election in 2011. He was later released without charge.
Gendarmes in Niger have arrested at least four senior military officers in a troubling sign of potential unrest in the West African nation just two months before elections, military sources and family members of those detained said on Thursday.
The arrests were made on Tuesday. Among those taken into custody were the former military chief of staff General Souleymane Salou and Lieutenant-Colonel Dan Haoua, the head of the air force base in the capital Niamey.
Nigerien authorities have given no reason for the arrests.
"It's said they are suspected of wanting to carry out subversive activities, but nothing is clear for the moment," said a relative of General Salou.
Political tensions are on the rise in Niger, a major uranium producer and an important Western ally in the fight against regional Islamist militants including Nigeria's Boko Haram.
The government on Tuesday fixed Feb. 21 as the date for presidential and parliamentary elections. President Mahamadou Issoufou is heavily favoured to win re-election, but the opposition accuses him of a campaign of repression ahead of the vote.
Issoufou's 2011 election marked the restoration of democratic rule after a military coup toppled his predecessor Mamadou Tandja a year earlier.
General Salou was a member of the junta that orchestrated the military takeover. Another of those currently being held in custody, Lieutenant Mahamadou Hambaly, was arrested on suspicion of preparing a coup against Issoufou just months after his election in 2011. He was later released without charge.