Ten generals and five other senior military officers have been found guilty in courts-martial of providing arms and information to Boko Haram, several Nigerian newspapers have said, though the military denied the sentences.
The reports follows months of allegations from politicians and soldiers who told the AP news agency that some senior officers were helping the armed group and that some rank-and-file soldiers even fight alongside the rebels and then return to army camps.
They have said that information provided by army officers has helped Boko Haram to ambush military convoys and in attacks on army barracks and outposts in their northeastern stronghold.
[Defence headquarters] wishes to state once again categorically that there is no truth whatsoever in the report.
The Leadership newspaper quoted one officer as saying that four other officers, in addition to the 15, were found guilty of "being disloyal and for working for the members of the sect".
Defence Ministry spokesman Major General Chris Olukolade, who last week denied reports that senior officers were being investigated, reiterated in a statement on Tuesday that defence headquarters "wishes to state once again categorically that there is no truth whatsoever in the report".
He called it a "falsehood" concocted by those who "appear hell-bent on misleading Nigerians and the international community to give credence to the negative impression they are so keen to propagate about the Nigerian military".
Nigeria Interior Minister Abba Moro told the BBC the courts-martial were "good news".
The army often denies substantiated reports, such as on extrajudicial killings of civilians and prisoners.
It is accused of such gross human rights violations that Washington's efforts to help in the rescue of nearly 300 abducted schoolgirls have been limited by US law restricting the sharing of some types of information and technology with abusive security forces.
The alleged sabotage by senior officers could explain the military's failure to curb a five-year-old uprising by Boko Haram that has killed thousands despite a year-long state of emergency in the northeast.
Boko Haram has attracted international condemnation and UN sanctions since its April 15 abduction of more than 300 schoolgirls, of whom 272 remain captive.
Nigerian activists pressing the government to rescue the schoolgirls filed a complaint on Tuesday against a police ban on protests, although a police spokesman later denied that any ban had been imposed.
President Goodluck Jonathan said last year that he believed that some members of the military, and even of his own government, including some cabinet ministers, sympathised with Boko Haram or belonged to the group.
Jonathan in January fired his entire military command and weeks later replaced the defence minister.