LAGOS, March 18 (Reuters) - Five Nigerian crew and a barge were released five days after they were captured by gunmen in the creeks of the oil producing Niger Delta in southern Nigeria, a source at the company operating the vessel said on Tuesday.

The men were released unharmed on Monday and the barge was recovered with most of the building materials it was transporting at the time of the attack on March 12, the source said.

He said the release was the result of negotiations with the attackers and no ransom had been paid.

The barge is operated by Nigerian-German construction company Julius Berger, an arm of German group Bilfinger Berger AG <GBFG.DE>.

The crew and the barge were kept hidden in the creeks of Rivers state, which has been the epicentre of violence in the anarchic delta for more than a year, until the release.

The Niger Delta, home to Africa's biggest oil industry which exports about 2.1 million barrels per day, is plagued by militancy and crime. Piracy, abductions for ransom, armed robberies and crude oil smuggling are all common problems.

Attacks on oil industry vessels and other commercial ships have become increasingly frequent over the past two years, prompting seamen's unions to campaign for shipping firms to grant crew war-risk bonuses for operating in Nigerian waters. (Reporting by Estelle Shirbon and Austin Ekeinde)

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