N'Djamena - Some 2,000 troops from regional military powerhouse Chad headed to neighbouring Niger on Wednesday, where Boko Haram insurgents inflicted heavy losses in the town of Bosso last week, a military source said.
Chad is a leading member of a multi-national force fighting the Nigeria-based Islamists who have extended their attacks to neighbouring countries from their base in northern Nigeria.
The "heavily armed" soldiers will "search everywhere for Boko Haram," added the source, speaking on condition of anonymity.
The attacks began Friday against a military post in Bosso in Niger's Diffa region, killing 26 soldiers including two from neighbouring Nigeria.
A total of 55 insurgents from the Nigeria-based Islamist group were killed and "many" injured, according to Niger authorities.
Also read: WATCH: Boko Haram finally surrenders
"An estimated 50,000 people or so fled," UN refugee agency (UNHCR) spokesman Adrian Edwards told reporters on Tuesday.
Last year, Chad dispatched soldiers to Nigeria, Niger and Cameroon to fight Boko Haram. The regional multi-national force includes the four countries.
UN voices 'great concern'
Boko Haram's seven-year insurgency has left at least 20,000 people dead in Nigeria and made more than 2.6 million homeless, leading to calls for more support within the region.
The UNHCR spokesman said most of those fleeing the violence in Bosso had walked to Toumour, some 30 kilometres (19 miles) to the west.
A local journalist working with Radio Anfani told AFP on Tuesday he was sheltering in Toumour with no food, along with many others who fled the violence.
"The Boko Haram gunmen stayed in Bosso from 6:30 pm (1730 GMT) on Friday to 3:00 am on Saturday, burning the military barracks, police facilities and local administration office before looting shops and carting away food supplies," he said.
He said that the gunmen used heavy artillery which allowed them to overrun the town's garrison.
"They came in large numbers shouting 'Allahu Akbar' (God is Greatest)," he said.
The journalist, who declined to be named, said that he subsequently learned that the fighters returned later on Sunday and "engaged troops redeployed to the town in fierce battle".
The journalist claimed the militants had overrun the town but Niger's defence minister Assoumana Malam Issa has said that the military had regained control.
Edwards said some of those displaced had moved on from Toumour and were heading to the town of Diffa, around 140 kilometres west of Bosso, and northwards towards a camp for internally displaced people that is already nearing its maximum capacity of 10,000.
"The welfare of these people and others forced to flee the violence in Bosso is of great concern," he said.
The latest attack was among the deadliest by the jihadist group in Niger.