Nigeria's Boko Haram rebels carried out their first attack inside neighbouring Chad, targeting a village on the shores of Lake Chad as part of a widening insurgency that has sucked in four countries.
Chadian soldiers patrol in the Nigerian border town of Gamboru as part of the fight against Boko Haram
The Islamist fighters crossed the vast lake by boat under cover of darkness to attack the village of Nougboua, across the water from the Nigerian town of Baga.
"They started firing on everything that moved," Chadian army spokesman Azem Bermandoa Agoun told national radio.
Two-thirds of Ngouboua, which has become a sanctuary for Nigerians fleeing attacks by Boko Haram, was torched in the onslaught, a security source said.
Chadian forces, backed by military aircraft, returned fire, routing the militants and destroying their vessels, the source said.
Chadian officials reported one civilian -- the village chief -- and one soldier killed, with a further four troops wounded. Two Boko Haram fighters were also killed and five injured, N'Djamena said.
The security source, who spoke on condition of anonymity, gave a higher Chadian death toll of four civilians, including the chief, and one soldier.
The attack marks a new escalation in Boko Haram's bloody six-year campaign to establish a hardline Islamic caliphate in northeast Nigeria, which borders Cameroon, Chad and Niger.
The group has killed thousands of people since 2009. In the past few weeks it has stepped up its offensive both within Nigeria and against border towns of neighbouring countries, forcing Nigerian general elections that were scheduled for February 14 to be postponed by six weeks.
- Unprecedented joint fightback -
Faced with the growing regional threat from the jihadists, Chad, which had been spared from a Boko Haram attack on its soil until Friday, has been at the forefront of a fightback.
Nigeria, Cameroon, Chad and Niger this month launched an unprecedented joint effort to crush the uprising, raising hopes that the insurgents -- who have outgunned Nigeria's national army -- might finally have met their match.
But the UN envoy for West Africa said Friday that Nigeria must step up and show "robustness" in the campaign.
"They have to demonstrate greater resolve than has been the case so far in this fight against a serious enemy," said Mohammed ibn Chambas.
On February 3 Chadian forces launched a ground intervention in the Nigerian border town of Gamboru.
The Chadian forces succeeded in wresting the town from the Islamists but were left reeling a day later after Boko Haram carried out a retaliatory attack in Fotokol, on the Cameroonian side of the border, in which 19 Chadian and Cameroonian troops as well as 81 civilians died.
The Islamists followed up last week with their first deadly raids in Niger, to Nigeria's north.
The militants repeatedly struck the border town of Diffa, after Niger announced it planned to send troops to Nigeria to combat them.
While engaging regional forces on several fronts Boko Haram also continues to eye the key prize of the key northeastern Nigerian city of Maiduguri, capital of Borno state.
At least 21 people were killed in two separate attacks Thursday on northeastern villages near Maiduguri, community leader Mustapha Abbagini and a witness said.
The rebels set fire to homes and businesses, Abbagini said.
Mbuta resident Hamidu Bukar said the gunmen accused locals of "spying for military authorities" and vowed to press on towards Maiduguri, where the group was founded in 2002 and which it has attacked at least twice since January.
- Pre-election violence -
Also Thursday, a female suicide bomber blew herself up at a crowded market in the town of Biu, in southern Borno state, killing at least 11 people according to a hospital source and a vigilante helping the army.
The death toll could increase further, with health officials working to establish the identities of at least two other people blown apart by the blast.
The violence in the north has loomed large over Nigeria's election campaign, adding to tensions in what is shaping up as one of the closest races in years, pitting struggling incumbent President Goodluck Jonathan against Muhammadu Buhari, a former military ruler who has vowed to vanquish Boko Haram.
Nigeria's human rights commission reported Friday that 58 people had been killed in pre-election violence and warned that rising "hate speech" between the rival camps threatened a "significant" escalation.
Africa's most populous country has a long history of bloodshed after elections but the pre-election violence was "atypical of Nigeria's recent electoral history," the commission said.
Isolated incidents of deadly unrest between rival factions, sometimes within the same party, were reported across the country.
The rights commission warned that, in the absence of "urgent steps" to defuse the political tensions, the elections, now set down for March 28, "would confront a high risk of significant violence."