Morris Alagoa, an activist for Niger Delta Rights, a human rights group in southern Nigeria, said Jones Abiri had been held by the Department of State Services (DSS).

Abiri was arrested at gunpoint on Sunday as he met colleagues at the offices of the Bayelsa Federated Newspaper Publishers' Association in the state capital, Yenagoa.

On Monday morning Alagoa said he spoke to Abiri, who confirmed he was "on the premises of the DSS in Abuja" before his subsequent release after his bail conditions were met.

Following his release, Abiri "still wasn't told his offence," according to Alagoa. "We are yet to be briefed on reasons for his arrest," he said.

Abiri, who is the editor and publisher of the Weekly Source newspaper, had previously been detained by intelligence officers for two years without trial.

He was held over alleged links to armed rebel groups in the oil-rich Niger Delta region.

During his detention he had no access to a lawyer or his family, according to the Committee to Protect Journalists (CPJ).

Abiri was freed in August last year following a campaign by rights organisations.

CPJ Africa programme coordinator Angela Quintal said: "We are deeply worried that he has once again been arrested and that his whereabouts are not known.

"We call on federal and state authorities in Nigeria to disclose where Abiri is being detained and the reasons for his arrest, and urge that they ensure that his rights are not violated yet again and that due process is respected."

Amnesty International also expressed concern over the arrest and said "the humiliating manner of his arrest is unacceptable".

Reporters Without Borders places Nigeria in 119th place out of 180 on its World Press Freedom Index.

It says journalists "are often threatened, subjected to physical violence, or denied access to information by government officials, police, and sometimes the public itself".