The former number two of Niger's junta, Colonel Abdoulaye Badie, was arrested late Friday, his family said, despite an earlier claim by the head of state that no government member had been detained.
Family members told AFP that Badie was arrested around 6:30 pm (1730 GMT) at his home.
"We do not know where he is now. We are worried," said an associate, speaking on condition of anonymity.
Another source said he was being detained at a gendarmerie in Niamey, capital of the uranium-rich west African state.
Badie had been permanent secretary to Salou Djibo, leader of the coup that overthrew Niger's president Mamadou Tandja in February, but the junta leader abolished the post.
A west African official earlier said that Djibo had told regional representatives that no arrests had been made.
Rumours of a coup have swirled in Niamey over the past few weeks. For several days, the military presence in the capital and its suburbs has been stepped up, with larger patrols, particularly at night, AFP journalists noted.
The junta has pledged to hand power to a democratically-elected civilian government. The transitional process is due to kick off on October 31 and culminate with a presidential election on January 31, 2011.
The planned handover is set for April 6, 2011, when the new president is due to be sworn in.
A member of the ruling junta, colonel Amadou Diallo, was removed Friday from his post as equipment minister, according to an official communique read on national radio.
The reasons for the removal were not given.
On October 8, Diallo was removed from his post as the prosecutor before the military tribunal.
The last civilian president, Tandja, was ousted by the military after he took a series of steps to prolong his term in office beyond the end of his elected mandate, which was in December last year.
The junta won widespread public support in February when it pledged to turn the west African country, which ranks last on the Human Development Index, into a beacon of "good democracy and governance".
The junta has thus far refused to release Tandja, whom it is holding in a villa within the presidential compound.© ANP/AFP