Nigerian militant groups have splintered into factions
The main militant group in Nigeria's oil-producing Niger Delta has said it is calling off a four-month ceasefire after a military attack on a camp.
The Movement for Emancipation of the Niger Delta (Mend) said the attack was repulsed, and several people killed.
It was on a camp run by the Niger Delta Vigilantes (NDV) - a group affiliated with Mend - and lasted more than an hour, militants said.
Mend announced the ceasefire after a two-week "oil war" in September.
The military said they were fired on first.
"The firing was from a distance in the surrounding creeks and our own troops responded," military spokesman Lt Col Sagir Musa said.
He said one soldier had been wounded.
Oil production has been cut in Nigeria by about a fifth since 2006 due to militant attacks on oil infrastructure.
Mend is the most publically visible group, but there are a number of other factions that operate under its umbrella, and engage in shifting alliances.
An NDV spokesman told Reuters news agency that the group had sunk one of the gunboats that attacked their camp.
The fighting could not be independently verified.
Mend promised a "sweeping assault" on the oil industry.
Mend have announced an end to the ceasefire before, following the killing in January of a militant arrested by the Joint Military Task Force in charge of security in the Delta.
This time they said the ceasefire would end at 0100 local time on Saturday.
Violence has continued in the Delta during the Mend ceasefire, including attacks by Mend "affiliate" organisations.
The announcement comes at a time when violence and kidnapping by armed gangs of jobless youths is on the rise.
Mend say they are fighting a political battle to get more money from oil to the people of the Niger Delta.
The Niger Delta Vigilantes, and their leader Ateke Tom, have been in and out of allegiance with Mend over the last three years.
There are several armed gangs who operate in the Niger Delta that make money through kidnapping and extortion, without any apparent political motive.
On Thursday an 11-year-old girl was killed as she tried to protect her brother from being kidnapped and on Sunday armed men kidnapped a catholic priest, who was later released.
The announcement of the end of the ceasefire might be an attempt by Mend to focus attention back onto the group, correspondents say.