Gunmen ambushed an oil supply vessel and tugboat in two separate incidents on Friday, a military spokesman said, the latest in a series of attacks off the coast of Nigeria's Niger Delta region.

Africa's top oil producer has one of the world's highest incidence of piracy, second only to Somalia, with 10 attacks reported so far this year.

Ten armed men in two speedboats early on Friday attacked the MV Ngoni, an oil supply vessel contracted by U.S. oil major Exxon Mobil <XOM.N>, as it was travelling through the busy waters near Bonny in Rivers state.

There were no fatalities or kidnappings, but some items were stolen from the vessel, said a security source working in the oil industry.

Lieutenant Colonel Sagir Musa, spokesman for the military task force in Rivers state, said he did not believe the attack was near any oil facility.

Insecurity in the region has cut Nigeria's oil output by around a fifth and has led foreign companies in sectors from construction to telecoms, as well as the oil industry, to reconsider investments and reduce expatriate staffing.

In a separate incident, 20 gunmen in three speedboats hijacked a tugboat near the same area as the attack on the Exxon vessel.

Musa said no one was hurt in the incident and the tugboat was later recovered.

"The attacks of yesterday and today show that the oil services companies must liaise with the (military) as we have dedicated gunboats to escort them on their movements," Musa said. "We are begging them to come to us."

The Niger Delta has long been plagued by violence but the number of attacks on vessels this year and abductions in areas previously considered relatively safe has alarmed the security forces.

Last year, there were 40 reported incidents of piracy in Nigeria, including 27 vessels boarded, five hijackings and 39 crew members kidnapped, according to the International Maritime Bureau.