The five Indian crew members of an oil tanker who were kidnapped last month after heavily armed pirates stormed their vessel off Nigeria's coast have been released, their employer said Saturday.

Medallion Marine, a Mumbai-based shipping firm, said the hostages were freed in good health, but did not disclose whether a ransom had been paid, or whether Nigeria's security forces played any role in securing their release.

"The five crew members who were taken from the (MT SP Brussels) vessel by armed men off the Niger Delta in December have been released," a statement said.

"Following medical examinations, the crew members have now been flown home in order to be reunited with their families," it added.

After the December 17 attack, a company spokesman told AFP that all five people kidnapped were Indian nationals.

Kidnap for ransom is common in Nigeria's oil producing Delta region, with expatriate workers often being targetted.

A 2009 amnesty deal with rebel groups in the area significantly reduced such incidents, but they still sporadically occur, while the companies involved rarely disclose details concerning ransom demands.