LAGOS, Nigeria — An anti-piracy watchdog says four sailors have been kidnapped off the coast of Nigeria's oil-rich southern delta.

The International Maritime Bureau said the kidnapping happened Sunday night on an offshore tug boat near Nigeria's Bayelsa state. The bureau said the gunmen seized four crew members before leaving.

The bureau said the ship has since reached a safe harbor.

An initial report to private security officials in Nigeria, separate from the bureau report, said the four sailors were foreigners. The bureau said it had no other immediate information about the attack. A Nigerian naval spokesman declined to immediately comment Monday.

Piracy has grown off Nigeria's Niger Delta and elsewhere in West Africa's Gulf of Guinea in recent years.