Two supply vessels have been attacked off the coast of Nigeria within 24 hours, with one vessel being briefly hijacked by pirates before being released. Piracy off West Africa has increased significantly this year, a maritime watchdog has warned.

On May 7 a supply vessel with 17 crew on board was hijacked approximately 40 nautical miles from the coast and held for around 11 hours before being released, while the next day six pirates armed with assault rifles boarded a supply vessel towing a barge off Pennington Oil Terminal, Nigeria. The pirates launched their speedboat from a fishing trawler.

The crew locked themselves in the ship’s citadel and emerged after 1.5 hours to find the pirates had gone, after breaking the bridge windows and stealing some of the ship’s properties, the International Maritime Bureau (IMB) reports. “Generally all waters in Nigeria remain risky. Vessels are advised to be vigilant as many attacks may have gone unreported,” the Bureau warns.

Attacks in the Gulf of Guinea have increased in recent months as the area, spanning a dozen countries, is a growing source of oil, cocoa and metals being shipped to the world's markets. The IMB said there were ten attacks off Nigeria in the first quarter of this year. A further attack in neighbouring Benin was also attributed to Nigerian gangs.

"Nigerian piracy is increasing in incidence and extending in range," said IMB director Pottengal Mukundan. "While the number of reported incidents in Nigeria is still less than Somalia, and hijacked vessels are under control of the pirates for days rather than months, the level of violence against crew is dangerously high."

Meanwhile, the Nigerian Maritime Administration and Safety Agency (NIMASA) recorded a total of 83 piracy and sea robbery attacks, including other unlawful acts at sea, in 2011.

The attacks were mostly recorded from the country’s high risk areas of Bonny, Calabar, Port Harcourt, Warri and Lagos pilotage districts. Lagos pilotage district accounted for six of the reported attacks that resulted to varying degrees of injuries to ship crew, loss of lives of seafarers, loss of cash and shipboard equipment, Nigeria’s Business Day reports.

Patrick Akpobolokemi, director general of NIMASA, said that his agency and the Nigerian Navy had established the Maritime Guard Command for regular patrol of Lagos waters, while the Nigerian Marine Police patrol inland waterways. In addition, NIMASA is partnering with the Nigerian Air Force to establish a maritime air unit at Benin to carry out surveillance.

Meanwhile, pirates on Thursday hijacked the MT Smyrni, a Greek-owned oil tanker carrying 135,000 metric tonnes (148,812 tons) of crude oil while in the Arabian Sea off Oman. The vessel was hijacked by ten armed pirates in two skiffs, who now have the 26 crewmembers hostage.

Seaborne gangs are making tens of millions of dollars in ransoms, and despite successful efforts to quell attacks in the Gulf of Aden, international navies have struggled to contain piracy in the Indian Ocean and Arabian Sea owing to the vast distances involved.