Maiduguri - Suspected Boko Haram militants killed 24 people in two separate attacks, eyewitnesses said on Saturday, just as the military vowed to ramp up security over Christmas.
Seven fishermen were ambushed and killed in the first attack in Baga, a fishing community on Lake Chad in Borno state, one of three in the northeast of the country under emergency rule since May this year.
Seventeen people died in a separate attack on Thursday, other eyewitnesses said, when gunmen in pick-up trucks torched more than 100 shops and vehicles in the Sabon Gari area of the Damboa district, 90 kilometres (56 miles) from the state capital, Maiduguri.
There was no immediate confirmation of either attack from the military or local authorities.
"They killed seven fishermen, injured 15 others and burnt some local boats and nets used for catching fish," fisherman Ibrahim Gambo said in Maiduguri, where he had brought his brother for treatment.
"It was a reprisal by insurgents because the soldiers have two weeks ago clamped down on them," added another fisherman, Sheriff Bababa.
"They (the military) even arrested some of their members, including a leader, with the assistance of youth vigilantes."
Human Rights Watch on Friday said Boko Haram fighters were carrying out reprisals on civilians in retaliation for intelligence on supposed militant activity passed to the military by civilian vigilante groups.
It urged the militants to stop targeting civilians and the vigilantes to stop using minors in counter-intelligence and security operations.
In Sabon Garin, villagers told reporters that the attack happened at about 11:35 pm (2235 GMT) on Thursday and those responsible were chanting "Allahu Akbar" (God is great) as they arrived.
The violence lasted until about 4:00 am, they added.
News of the attacks, which are often slow to emerge because of a communications black-out in Borno designed to disrupt militant planning, came as the military pledged to step up security in vulnerable border areas.
Boko Haram, which wants to impose a strict form of Islamic law or sharia on Muslim-majority northern Nigeria, has previously launched deadly attacks on and around the Christian festival.
A wave of attacks against churches and police on December 25, 2011, left 49 people dead.
Nearly 200 people, including soldiers, insurgents and scores of civilians, were killed in fierce fighting between troops and Islamist insurgents in Baga in April this year.
Area army spokesman Colonel Mohammed Dole said troops had been deployed to frontier villages and towns in Borno state that have been targeted while suspected Boko Haram bases were being cleared, backed by air support.
"We have identified their hideouts and we are determined to make all the border communities and the state generally free of Boko Haram activities so that people can move freely and celebrate the Yuletide peacefully," he added.
Borno state deputy governor Zanna Umar Mustapha has said the military would now set up permanent bases in trouble spots, which have shifted from urban centres to the countryside as a result of emergency rule.
National army spokesman Brigadier General Ibrahim Attahiru meanwhile said that Nigeria was seeking regional help from its neighbours to help put down the insurgency.