The United Nations on Tuesday said that the latest wave of Boko Haram's "vicious, ruthless attacks" in northeastern Nigeria had sent 11,320 people fleeing into Chad in a matter of days.

The Islamist group stormed the town of Baga on January 3, and subsequently razed it and at least 16 surrounding settlements.

A spike in Boko Haram attacks in northeastern Nigeria has sent some 7,300 people fleeing to neighbouring Chad in a matter of days, the U.N. refugee agency said on Friday.

Members of the armed Islamist movement on Saturday captured the town of Baga in Borno state.

The world is losing its ability to prevent conflicts and the lack of "effective leadership" has led to the worst displacement situation since World War II, the U.N. refugee chief warned Tuesday.

Antonio Guterres, the U.N. High Commissioner for Refugees, said in Ankara that the twin crises in Iraq and Syria had created a serious displacement situation and the world showed no effective leadership to address the challenges.

Suspected Boko Haram militants have seized a military base in northeastern Nigeria in a series of blistering raids near Lake Chad, sending soldiers and civilians fleeing, witnesses said on Sunday.

Local residents reported that the gunmen killed several people, burnt hundreds of homes and looted scores of businesses in Saturday's attacks, although there was no official death toll.

Leaders in the sub-Saharan Sahel region of Africa called Friday on the United Nations to organize an international force "to neutralize the armed groups" sowing chaos in Libya.

Boko Haram is increasingly a regional threat and the battle against the Nigerian Islamist sect is meant to be a regional campaign -- but that's not the way it feels for Cameroon's soldiers on a desperate frontline.

"We are fed up with fighting this war all alone," a Cameroonian officer said as he described his army's resistance against Boko Haram and the lack of military support from neighbouring governments.

Dozens of Chadian troops from the United Nations peacekeeping force in Mali have deserted their posts in a dispute over pay and conditions, military sources and the soldiers themselves told AFP on Thursday.

The soldiers, who are armed, left their station in the northern town of Aguelhoc on Tuesday night, some complaining that they hadn't been paid for up to four months, several sources told AFP.

French Prime Minister Manuel Valls arrived in Chad late Friday at the start of a two-day regional visit, in a show of support for a French-led anti-jihadist force.

In N'Djamena on Saturday and Niger's capital Niamey on Sunday, Valls will hold talks with respective presidents Idriss Deby and Mahamadou Issoufou.

Nigerian Islamist extremists Boko Haram are intensifying attacks in neighboring Cameroon, targeting new villages with increasingly sophisticated weapons, as the army fears more violence in the approaching dry season.

"We're convinced that the establishment of a 'caliphate' (by Boko Haram) is aimed not only at Nigeria but also at Cameroon," Leopold Nlate Ebale, commander for an elite battalion in the border zone, told Agence France Presse.

Twenty nine aides to Chad's ousted dictator Hissene Habre went on trial Friday before a special court in N'djamena charged with mass murder and torture during his rule in the 1980s.

In the very first trial inside Chad arising from the "black years" between 1982 and 1990, when thousands of people were killed or grievously hurt, the top suspect was Saleh Younous, once head of the Directorate of Documentation and Security (DDS), which was Habre's political police.