Armed pirates who stormed an oil service ship off Nigeria have kidnapped six foreigners and demanded a $1.3m ransom for their release, the police and military have told the AFP news agency.

"Three of those abducted are from Ukraine, two from India, one from Russia," Bayelsa state police spokesman Fidelis Odunna said of Sunday's attack.

"One of the kidnappers called to demand the sum of 200 million naira ($1.3m)," speaking on Wednesday.

The kidnapping of foreign oil workers is common in Nigeria's oil-rich south, with the hostages often released following a ransom payment. It is however rare for police to discuss the details of ransom demands.

There has also been a recent series of kidnappings in northern Nigeria claimed by an armed group, but those are considered a different phenomenon.

The Armada Tuah vessel, operated by the Lagos-based Century Group with a crew of 15, was attacked by gunmen 40 nautical miles off southern Bayelsa state at about 4:00 pm (1500 GMT), the area's military spokesman, Colonel Nwachukwu Onyema told AFP.

'Search and rescue'

The vessel was on route from the oil-hub of Port Harcourt to Abo, the site of a key deepwater oil field, where Shell, Italy's Agip and several domestic firms have operations, Onyema further said.

"Gunboats [and] troops are already on search and rescue," he added.

The International Maritime Bureau, which tracks sea attacks worldwide, has described the waters off Nigeria as an emerging piracy hub, with many of the raids involving gunfire.

Armed groups in Nigeria's oil-producing Niger Delta region carried out scores of kidnappings before a 2009 amnesty deal led to a sharp decline in unrest, though incidents continue.

Five Indian crew members of an oil tanker who were kidnapped in December after heavily armed pirates stormed their vessel off Nigeria's coast were released last month.

Medallion Marine, a Mumbai-based shipping firm, said the hostages were freed in good health, but did not disclose whether a ransom had been paid, or whether Nigeria's security forces played any role in securing their release.

The theft of crude is considered a key motivation for pirates operating in the Niger Delta.