Chadian troops deployed to fight Boko Haram in Nigeria have returned to Cameroon, where they are expected to be regroup for fresh offensives in border areas, military sources said on Friday.

Some 2,500 troops deployed as part of a regional force against the Islamist militants have in recent weeks retaken the towns of Gamboru and Dikwa, in Borno state, northeast Nigeria.

Iraqi forces Friday battled Islamic State jihadists making what looked increasingly like a last stand in Tikrit, but the group responded by vowing to expand its "caliphate".

Thousands of fighters surrounded a few hundred IS holdouts, pounding their positions with helicopter and artillery strikes but treading carefully to avoid the thousands of bombs littering the city center.

The Islamic State group has welcomed a pledge of allegiance to it made by the Nigerian jihadist organization Boko Haram, according to an audiotape Thursday purportedly from its spokesman.

"We announce to you to the good news of the expansion of the caliphate to West Africa because the caliph... has accepted the allegiance of  our brothers of the Sunni group for preaching and the jihad," IS spokesman Mohammed al-Adnani said in the message, using the Arabic name for Boko Haram.

U.N. Security Council diplomats are drafting a resolution to shore up a regional force fighting Boko Haram with financial backing and logistical support needed for the mission to succeed, according to a text obtained by AFP.

Under the measure, the Security Council would also consider sanctions against financiers and other supporters of the Nigerian Islamist group, which last week pledged allegiance to the Islamic State group.

The Nigerian Army and its allies have claimed a series of successes against Boko Haram, on the face of it handing President Goodluck Jonathan a trump card in upcoming elections.

But experts say the apparent territorial gains may not translate into votes at the ballot box on March 28, with the situation still uncertain on the ground and tensions between the militaries involved.

Nigeria said on Wednesday that 36 towns had been retaken from Boko Haram since the start of a four-nation military offensive, voicing hope that the operation could lead to the group's "total defeat".

National security spokesman Mike Omeri said four towns had fallen since last Friday, including three in Borno state and Buni Yadi, in neighboring Yobe, where the insurgents slaughtered more than 40 students in February last year before seizing it in August.

Seven people were killed in a suspected suicide bomb attack at a market in the northeast Nigerian city of Maiduguri on Tuesday, police said.

"Seven persons died while 17 others were injured. The rescue is still ongoing," Borno state police commissioner Clement Adoda told reporters.

The Red Cross on Tuesday warned of an increasingly dire situation in Nigeria and other countries circling Lake Chad, where hundreds of thousands have been forced to flee Boko Haram violence.

"There is a full blown humanitarian crisis around this lake, not only in Nigeria but also in the surrounding countries," said Jean-Yves Clemenzo, spokesman for the International Committee of the Red Cross.

Boko Haram's pledge of allegiance to the Islamic State group was a sign of weakness and a result of pressure on the jihadist militants by Nigeria and its allies, the government in Abuja said on Tuesday.

National security spokesman Mike Omeri called the pledge "an act of desperation and comes at a time when Boko Haram is suffering heavy losses".

The armies of Niger and Chad on Sunday launched a major ground and air strike against Boko Haram in northeastern Nigeria, opening a new front in regional efforts to eradicate the Islamist militants. 

The offensive announced by a source in the Niger government came after Boko Haram leader Abubakar Shekau pledged allegiance to the Islamic State group in an audio message.