Six crew members have reportedly been kidnapped off a Greek-owned tanker anchored in the waters off Nigeria.

The 3,075-dwt Apecus (built 1973) was boarded by intruders 19 April off Bonny Island, roughly 30 miles southeast of Port Harcourt, maritime security consultant Gray Page said in an alert Wednesday.

The Apecus is owned by Tomazos Shipping and managed by Petrogress subsidiary Petronav Carriers.

No one at Petrogress was available for comment.

The alert said there were reports that the crew members were Indian nationals and that the attack happened during daylight hours. The remaining crew members are said to be safe.

The attack was the second on an anchored ship in the region in less than a week. On 17 April, pirates boarded at an arrested ABC Maritime product tanker, but fled after Nigerian Navy guards fired.

Gray Page cautions vessels in the Bonny Island area to exercise "extreme caution," keep strict watches and monitor nay unexpected approach by small craft.

In its first quarter report, the International Maritime Bureau (IMB) said all 21 crew members kidnapped worldwide were taken in the Gulf of Guinea.

While the IMB credited the Nigerian Navy for its efforts to fight piracy, but cautioned that Nigeria waters were still dangerous for ships and seafarers.