Boko Haram has suffered an internal split after a group rebelled because leader Abubakar Shekau failed to adhere to guidance from Islamic State, a senior US general said on Tuesday.

There is a concern that the splinter group might work more closely with ISIS, including by adopting its transnational focus, which has been absent from Boko Haram so far.

Marine Lieutenant General Thomas Waldhauser, the nominee to lead the US military's Africa Command, suggested the internal division was illustrative of limits of ISIS' influence over Boko Haram so far, despite the West African group's pledge of allegiance to it last year.

"Several months ago, about half of Boko Haram broke off to a separate group because they were not happy with the amount of buy-in, if you will, from Boko Haram into the ISIL brand," Waldhauser said at his nomination hearing before the Senate Armed Services Committee.

Shekau, he said, had not fallen into line with Islamic State's instructions, including by ignoring calls for Boko Haram to stop using children as suicide bombers.

"He's been told by ISIL to stop doing that. But he has not done so. And that's one of the reasons why this splinter group has broken off," he said, adding Islamic State was trying to "reconcile those two groups."

Reuters reported on June 9 that US officials had seen no evidence that Boko Haram has so far received significant operational support or financing from Islamic State. The assessment suggested Boko Haram's loyalty pledge had so far mostly been a branding exercise.

Waldhauser acknowledged differing opinions about how much influence Islamic State has actually had so far over Boko Haram, which won global infamy for its 2014 kidnapping of more than 250 schoolgirls.

"They certainly have not given them a lot of financial assistance. So the point being is that perhaps improvement in tradecraft, in training and the like," he said.

While it is estimated to have killed more than 15,000 people since 2009, Boko Haram has not attacked US interests and has deep roots in Nigeria's Christian-Muslim divide, which long predates the Syrian-based Islamic extremist group.

Waldauser noted Shekau's local focus and voiced concern about whether a splinter group might act more in concert with Islamic State's transregional ambitions.

"What concerns me is the breakoff group of Boko Haram who wants to be more ISIL-like, and consequently buy into the ISIL-brand of attacking Western interests," he said.

"That would concern me."

The Nigerian army is making significant inroads in its offensive against Boko Haram. On June 3, 245 hostages were rescued from the Islamist militant group. Last month, more than two years after 276 Chibok schoolgirls were abducted by Boko Haram, two girls were recovered among a further 97 abductees, raising hopes for the remaining group.

On May 14, French President Francois Hollande told a security summit in Nigeria that "[Boko Haram] has been weakened, it's been pushed back, it's been chased around and has given up the territories it was controlling, and as a result it's being even better targeted and fought. However, this terrorist group remains a threat."

Additional reporting by Reuters.