Nigeria-based Boko Haram is responsible for countless atrocities and thousands of deaths, and it has declared itself an affiliate of Islamic State. But according to US officials, that's little more than a branding exercise designed to boost its international jihadi credentials, attract recruits, and appeal to the IS leadership for assistance.

Officials have told Reuters they see no evidence that Boko Haram has received significant operational support or financing from Islamic State, more than a year after the brutal West African group's pledge of allegiance to it.

The implications are about more than terminology. While the Nigerian army under the direction of President Buhari, who won election in good part because of the ineffectiveness of his predecessor Goodluck Jonathan, has been making steady progress against the militants, they remain a potent and disruptive threat. It launched its deadliest raid in over a year last week, killing 30 soldiers and forcing 50,000 people to flee when it took over the Niger town of Bosso. Chad has sent 2,000 troops to Niger to prepare a counterattack against the group, two senior military sources said on Wednesday.

But the US view of Boko Haram as a locally-focused, homegrown insurgency is likely to keep the group more to the margins of the US fight against Islamic State in Africa.

The US military's attention is largely centered on Libya, home to Islamic State's strongest affiliate outside the Middle East and where the United States has carried out air strikes. No such direct US intervention is currently being contemplated against Boko Haram, officials say.

"If there is no meaningful connection between ISIL and Boko – and we haven't found one so far – then there are no grounds for US military involvement in West Africa other than assistance and training," said one US official.

"This is an African fight, and we can assist them, but it's their fight," the official added.

While Nigeria and Chad, also deeply involved in the struggle against Boko Haram, might welcome American help with surveillance and intelligence-gathering, they are not thought to be anxious for foreign boots on the ground.

However, some are keen for America to do more to help in the struggle.

US officials and private experts say they fear that as the African military pressure intensifies, the extremists could shift from a regional campaign of suicide bombings, rape and pillage to striking international targets.

"The resources and intent of ISIL to attack Western targets, combined with Boko's ability and strength in that part of Africa is a mix that causes great concern," another US official said.

Senator Chris Murphy, a Foreign Relations Committee member, said that whatever its cooperation with Islamic State, Boko Haram is so deadly that Nigeria and its neighbours should get US help to crush the group.

"I think we have an interest in combating this group regardless of their connection to ISIL," he said.

Boko Haram is one of a number of African Islamist movements. It came to world-wide attention with the mass kidnapping of 276 schoolgirls from a school in Chibok; most are still missing and believed to be either dead or forcibly "married" to Boko Haram fighters.

Additional reporting by Reuters.