DAMATURU, Nigeria (REUTERS) - Gunmen suspected to be allied to Islamist sect Boko Haram have stormed a construction site in northeast Nigeria, carting off 125kg of dynamite and hundreds of detonators, two security sources said on Wednesday.

The militants struck the Bara area of Yobe state at around 2am on Monday morning, the two security sources in Yobe told Reuters, but no shots were heard. One of the sources said the men arrived on four motorcycles, beating and overpowering the site's private security guards.

The police and military both declined to comment. Officials from the company, a Nigerian firm called Ric Rock, were not immediately available for comment.

The Nigerian military launched a major offensive in May aimed at defeating the Islamists in three northeastern states - Borno, Yobe and Adamawa - but that has pushed many of them into hiding, from where they can still launch deadly attacks.

Multiple bomb blasts in Nigeria's biggest northern city of Kano killed at least 15 people on Monday, underscoring the continuing threat the rebels pose in the north.

Since its initial uprising in 2009, Boko Haram has grown in sophistication, learning bomb-making technology from Al-Qaeda linked militants in the Sahara to the north.

Videos shown to Reuters by a counter-terrorism official on Sunday showed Boko Haram militants giving detailed instructions on how to make bombs - including a suicide vest and a car bomb - in the northern Hausa language, filming how it is done.