Chadian forces scored a key victory over Nigeria's Boko Haram on Tuesday, wresting control of the border town of Gamboru within hours of launching a ground offensive against the jihadist group.

Around 2,000 Chadian troops backed by armored vehicles crossed the border into Gamboru from the Cameroonian town of Fotokol after days of clashes with the insurgents.

Nigeria's military said on Tuesday said the country's sovereignty was not compromised despite the presence of Chadian ground troops in the northeast to fight Boko Haram.

Chadian soldiers crossed the border from northern Cameroon to the Nigerian town of Gamboru on Tuesday after three days of bombardment of Boko Haram positions in a regional fight-back against the Islamists.

Chad sent ground troops into Nigeria for the first time on Tuesday to fight Boko Haram in a sign of increased regional pressure on the Islamist rebels after weeks of surging violence.

The Chadian offensive on the town of Gamboru, which borders Cameroon, follows days of clashes with the Islamists who control vast swathes of northeast Nigeria.

Two female suicide bombers were suspected of being behind a blast on Monday outside a stadium in northeast Nigeria, where President Goodluck Jonathan had given a speech, officials and rescue workers said.

"We have evacuated two bodies of females we believe were suicide bombers behind the blast," said one rescue worker after the explosion in Gombe city.

Nigeria on Monday braced for fresh Boko Haram attacks ahead of this month's elections, with the key city of Maiduguri in the firing line and forces from Chad and Cameroon joining the regional fight.

A weekend of violence saw the Borno state capital, Maiduguri, hit for the second Sunday in a row, but Nigerian Army soldiers, helped by civilian vigilantes, managed to keep the militants at bay.

Nigeria's military on Sunday repelled a Boko Haram assault on the key city of Maiduguri as violence raged across the country's northeast just two weeks before national elections.

The hours-long attack on the strategic capital of Borno state was the Islamists' second attempt to take Maiduguri in a week. 

Boko Haram fighters on Sunday launched a fresh attempt to take over the strategically crucial northeastern Nigerian city of Maiduguri, a brazen attack on a state capital just two weeks before national elections.

Four residents told AFP that Sunday’s fighting began at roughly 3:00 am (0200 GMT) with loud explosions and gunfire, as the Islamists tried to enter the city from the south.

Chadian aircraft on Saturday bombed the Nigerian town of Gamboru in a raid targeting extremist group Boko Haram, security sources said, as regional efforts to combat the Islamists intensified. 

An initial raid was carried out around midday by two fighter jets on the town in Nigeria's far northeast along the Cameroon border, sources from Chad and Cameroon said on condition of anonymity.

U.N. Secretary-General Ban Ki-moon on Saturday welcomed an African Union proposal to set up a regional five-nation force of 7,500 troops to fight Nigeria's Boko Haram Islamist militants.

"I welcome the decision of the AU and regional countries to establish an MJTF (Multinational Joint Task Force) against Boko Haram," he told reporters during an African Union summit being held in the Ethiopian capital.

Thousands of Nigerian civilians who fled the armed Islamists of Boko Haram have become a headache for Chadian authorities after seeking safety on a multitude of scattered islands in Lake Chad.

"We waded through the water for several days and suffered so much," recalled former taxi driver Adamou Bouba, who snatched up his toddlers, aged two and three, when Boko Haram fighters raided his village, Kiguili, and killed his wife.