Niger and Chad on Sunday launched major ground and air strikes in northeast Nigeria against Boko Haram, after the militants formally pledged allegiance to the Islamic state group in Syria and Iraq.

The attacks, which follow a sustained build-up of troops in southern Niger, opens up a new front in regional efforts to wipe out the Islamist group, whose six-year insurgency has spread across borders.

"Very early this morning, troops from Niger and Chad began an offensive against Boko Haram... in the area of Bosso and near to Diffa," a Niger government source in Niamey told AFP on condition of anonymity.

More than 200 vehicles, some of them with machine guns, as well as tanks, ambulances, water tankers and transport trucks, were seen moving towards the border, radio station Anfani, based in Diffa, said.

Aircraft targeted Boko Haram positions on Saturday and early Sunday, it added, while a Diffa resident and aid worker said troops were seen heading to the border and heavy gunfire was heard.

The previously lacklustre counter-insurgency against Boko Haram has been given renewed vigour with the deployment of troops from Cameroon, Chad and Niger last month.

The armies have claimed a series of successes in rebel-held territory in recent weeks, as part of an operation to clear and control northeast Nigeria in time for general elections set for March 28.

Much of the focus has been on Chad's well-trained army, who have experience in tackling Al-Qaeda-linked extremists in Mali alongside French forces.

But Nigeria's military spokesman Chris Olukolade said the Nigerien and Chadian attacks were "complementary to the ongoing push against the terrorists" that it was heading.

"The mission of routing the terrorists from Nigeria's territory is being conducted from various fronts in and out of Nigeria and is achieving expected results so far," he said in a text message.

- Game changer? -

More than 13,000 people have been killed and some 1.5 million made homeless in the conflict since 2009 and the coalition is a reflection of regional fears about security, following cross-border attacks.

The African Union on Friday endorsed the creation of an additional regional force of up to 10,000 men to join the fight against Boko Haram.

Western powers have so far largely stayed out of direct involvement in the conflict, viewing it as an essentially "local" or regional problem.

But Boko Haram's pledge of allegiance to Islamic State in an audio message on Saturday night, could be a game-changer, according to analysts.

"Operating as an IS proxy could draw Western efforts against its operations, particularly from France, which already runs a multi-country anti-terrorist campaign in the west and central African region," said Ryan Cummings, from risk consultants Red24.

"Operation Barkhane's headquarters in the Chadian capital N'Djamena is just a stone's throw away from Boko Haram's area of operations in northeast Nigeria," he added.

- Increasing links -

Boko Haram leader Abubakar Shekau described the formal pledge of allegiance to IS leader Abu Bakr al-Baghdadi as a religious duty and said it would "enrage the enemy of Allah".

In doing so the Nigerian militants join extremists from Libya to Pakistan who have previously done the same, as IS group looks to expand its reach.

There have been growing indications that Boko Haram was looking to formally align itself, starting last year when Shekau declared the captured town of Gwoza, in Borno state, part of a caliphate.

That followed a similar declaration by Baghdadi while both groups have captured swathes of territory.

In recent weeks, previously poorly produced Boko Haram videos took on the look and feel of more polished IS propaganda and were posted directly online, guaranteeing a wider audience.

IS was also reported to have received pledges of allegiance from militants in Nigeria.

Shekau may have attempted to tone down his violent rhetoric and improve Boko Haram's public image among the wider jihadi movement but indiscriminate attacks have continued against civilian "soft" targets.

On Saturday, at least 58 people were killed in three bomb blasts in the Borno state capital, Maiduguri, in a further example that the threat has far from been contained as elections approach.

Experts said the pledge of allegiance to IS, which is also being forced on the defensive in Iraq, could be a result of the military pressure.

"This might be a way to send a message to the troops, to strengthen morale or attract more followers, particularly in Boko Haram's case," said Yan St-Pierre, a counter-terrorism expert at the Modern Security Consulting Group in Berlin.

St-Pierre suggested the pledge was linked to IS inroads in Libya, from where Boko Haram has been said to receive arms and ammunition via smuggling routes in the Sahel region.

"Most of the contact Boko Haram groups or factions have had with ISIS have been along that axis, and Sudan," he added.