“It’s quite clear that since at least mid-January, the Islamic State has had some level of connection with Boko Haram,” said Aaron Y. Zelin, a fellow at the Washington Institute who tracks propaganda by Islamic extremists. “The key question is whether the Islamic State dispatched individuals from Syria or Iraq, or else from Libya, down to northern Nigeria to help out with operations on the ground, or else with methodology, or in terms of governance activities.”

Boko Haram is estimated to have up to 6,000 fighters and at least some level of control over about 20,000 square kilometers, or about 8,000 square miles, of northeastern Nigeria, according to Ryan Cummings, chief Africa analyst for Red24, a crisis management group based in Britain, who has been following the group since 2011. Mr. Cummings raised questions about the kind of command-and-control structure that could exist between the two groups.

“It seems at the moment that this is a statement that is akin to saying, ‘We are on the same page,’ ” he said. “But the biggest issue with Boko Haram is that it’s not a homogeneous group and it behaves as different factions. So it’s too early to tell for sure if Boko Haram will fall directly under ISIS command, and to what extent they will act as an ISIS proxy.”

Since its inception, Boko Haram’s targets, goals and language have been almost exclusively Nigerian, and the focus of its hate remains the Nigerian state and its agents.

Moreover, the group’s brutality is unlikely to be altered by a new alliance. Paul Lubeck, a Nigeria expert at Johns Hopkins University, said Saturday that Boko Haram had been practicing the signature tactics of the Islamic State — beheadings and enslavement — even before the Islamic State.