Pirates who kidnapped six foreign sailors from an oil servicing vessel off the Nigerian coast on Sunday are demanding 200 million naira ($1.3 million) for their release, police said on Tuesday.

One Russian, three Ukrainian and two Indian sailors were taken when gunmen stormed the Armada Tuah 40 miles (65 km) off the coast of oil-producing Bayelsa state.

The vessel is owned by Nigerian oil servicing company Century Group, Bayelsa Police Spokesman Fidelis Odunna said. The company did not respond to calls for comment.

"One of the kidnappers called to demand the sum of 200 million naira ... efforts are in progress to arrest the kidnappers," Odunna said, adding that nine Nigerian crew members had been left on board by the gunmen.

Piracy off the Nigerian coast and elsewhere in the Gulf of Guinea is on the rise. There have been at least five attacks in the waters off Africa's most populous nation this month.