The military says gunmen in Nigeria’s southern oil region have hijacked an oil-services vessel with 11 crewmembers onboard.

Military spokesman Maj. Sagir Musa says the unknown hijackers are demanding about $250,000 for the release of the boat and the crew, including one Portuguese and one Ukrainian.

He says the boat is used for maintaining oil infrastructure in Africa’s biggest petroleum producer. It was taken on Tuesday and that the standoff in the southern Niger Delta is continuing Wednesday.