Journalist Alexander Alesin "has not come to work for several days now," Irina Krylovich, deputy editor of Belarussians and the Market independent newspaper where the journalist works in Minsk, told AFP.
Colleagues said Alesin, who is well known as a commentator on defence issues and an expert on the arms trade, had not turned up for work yesterday or today and his cell phone was switched off.
Opposition politician Yury Chausov wrote on Facebook: "Alexander Alesin is in a KGB prison," referring to the security services, still known by their Soviet-era initials.
The opposition news site Charter 97 alleged that Alesin had been arrested on suspicion of espionage, citing colleagues who wished to remain anonymous.
Lithuania yesterday charged a military medic with spying for Belarus.