A Belarussian journalist who specialises in defence has been missing for several days, his colleagues said on Thursday, raising fears he has been detained by security forces.

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 on Wednesday or Thursday 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.

Chausov added the suggestion that Alesin "will be exchanged for someone," apparently referring to a prisoner exchange.

The opposition news site Charter 97 alleged that Alesin had been arrested on suspicion of espionage, citing colleagues who wished to remain anonymous.

Lithuania on Wednesday charged a military medic with spying for Belarus.