The Niger Delta Avengers group, which has been attacking pipelines and facilities since early this year, denied involvement but claimed a separate strike and vowed to bring crude output to a halt.
The deadly attack on the Nigerian National Petroleum Corporation (NNPC) boat happened at about 5:50 pm (local time) on Wednesday in the Warri area of Delta state, which has seen a spate of attacks recently.
"During the deliberate attack two soldiers were killed, one wounded, one soldier missing while four civilian staff attached to the house boat were shot dead," army spokesman Captain Jonah Unuakhalu said in a statement yesterday.
But the Niger Delta Avengers quickly said it was not involved.
Also Read
"Killing of sleeping soldiers is not our style," it said in an email.
"We promise the world that in this process of liberating our people, not a single blood of Nigeria soldiers will be wasted despite the provocation...
"Our war is on oil installations not to take innocent lives," it said.
The group earlier this week, however, warned it would step up its campaign with violence against facilities and personnel.
The group, whose targets have included oil majors such as Shell, Chevron, Eni and the NNPC, earlier claimed another attack.
"At about 2:00 am today @NDAvengers blew up the Ogboinbiri to Tebidaba and Clough Creek to Tebidaba Crude Oil pipelines in Bayelsa State," according to its Twitter account.
"This is in line with our promise to all international oil companies and indigenous oil companies that Nigeria oil production will be zero," it added.
"We know what they are doing," he told AFP, indicating that several arrests made last month had led to fresh intelligence.