Suspected Boko Haram gunmen armed with explosives and heavy weapons attacked a military barracks in Nigeria's troubled northeastern city of Maiduguri on Friday, witnesses said. The gunmen stormed the Fauri area of the city after crossing a river on the fringes of Maiduguri. They opened fire on the Giwa military barracks, sparking a fierce exchange of fire with the soldiers inside, according to several witness accounts. An AFP reporter near the barracks said the shooting started shortly after 7:00 am (0600 GMT). The gunmen tossed explosives into the military compound, but it was not clear if they were able to force their way in, he reported, adding that smoke could still be seen billowing out of the barracks. Area resident Dauda Mahmud said the Boko Haram gunmen "came in large numbers". Another witness, who requested anonymity told AFP that "explosions and gunfire could be heard throughout the city." "We have never heard such explosions in this city for a very long time," he added. Students at the University of Maiduguri reported hearing sustained gunfire throughout the morning. The campus is near the barracks and it was not clear if the university was separately targeted. "There have been several deafening explosions and unending gunfire for the past one hour," said female student Mercy Bitrus. "The fighting is no doubt taking place near Giwa barracks which is very close to us because the sounds are coming from the direction of the barracks," she added. The military in Borno state, where Maiduguri is the capital, did not answer repeated phone calls seeking comment. The Boko Haram conflict has killed thousands since 2009, with more than 500 dead already this year, mostly in Borno. Three states in the region -- Adamawa, Borno and Yobe -- have been under a state of emergency since last May when the military launched a major offensive to quash the Islamist uprising.