By Jonathan Landay, Warren Strobel and Phil Stewart

WASHINGTON (Reuters) - After Boko Haram killed more than two dozen soldiers in Niger last week, it claimed the attack in the name of Islamic State-West Africa Province -- a title meant to tell the world it is an arm of the Syria-based extremist group.

But U.S. officials tell 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.

That assessment, detailed by multiple U.S. officials who spoke on condition of anonymity, suggests Boko Haram's loyalty pledge has so far mostly been a branding exercise designed to boost its international jihadi credentials, attract recruits, and appeal to the IS leadership for assistance.

The U.S. view of Boko Haram, which won global infamy for its 2014 kidnapping of 276 school girls, as a locally-focused, homegrown insurgency is likely to keep the group more to the margins of the U.S. fight against Islamic State in Africa.

The U.S. 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 U.S. 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 U.S. military involvement in West Africa other than assistance and training," said one U.S. official, using an acronym for Islamic State.

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

In public comments, senior U.S. officials have said they are closely watching for any increased threat to Americans from Boko Haram and any confirmation of media reports of deepening ties with IS.

Despite suffering a series of setbacks, Boko Haram remains lethal. 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.

U.S. military action against ISIL in Iraq and Syria is conducted under legislation Congress passed after the Sept. 11, 2001, attacks and authorizes the use of American military power against "those responsible for" those attacks. As the Obama administration has interpreted it, that includes Islamic State as a third-generation descendent of Osama bin Laden's core al-Qaeda group, but not Boko Haram, said the official.

U.S. officials acknowledge their intelligence about the internal structure and leadership of Boko Haram is imperfect.

But the United States has closely tracked ISIL's leadership, finances and other activities, including its cooperation with other groups such as its branch in Libya, to which Islamic State has sent fighters, commanders and other support.

Multiple U.S. officials said they have seen no evidence that Islamic State leaders, based in Syria and Iraq, have transferred significant amounts of cash or weapons or sent high-level representatives to Nigeria.

The absence of such evidence comes as the administration of President Barack Obama debates how Washington and its allies can best support Nigeria and its neighbors. Some U.S. lawmakers already argue that U.S. aid to the region has been too heavily weighted toward security.

DEBATE ON ASSISTANCE

U.S. security assistance to the four African countries plagued by Boko Haram - Nigeria, Niger, Chad and Cameroon - has soared to more than $400 million since 2014, surpassing aid for governance, human rights, education and rebuilding infrastructure, according to a recent Congressional Research Service report.

The Obama administration is poised to approve the sale of 12 attack aircraft to Nigeria, Reuters reported last month.

The United States also has offered to send a Special Operations mission to advise Nigerian units, and has dedicated more intelligence and surveillance assets to help African forces fight Boko Haram.

Still, some U.S. government experts warn that defeating it requires Nigeria to boost policing, education and development in its Muslim-dominated northeast and to crack down on corruption.

Administration officials say that it's easier to win congressional support for military assistance to fight extremist groups - especially if defense contracts are involved - than it is to muster backing for steps to attack radicalism at its roots.

While it is estimated to have killed more than 15,000 people since 2009, Boko Haram has not attacked U.S. interests and has deep roots in Nigeria's Christian-Muslim divide, which long predates the Syrian-based Islamic extremist group.Those uncertainties have fueled tension over how best to combat the group, and even how to characterize it. In public, U.S. officials rarely call the group Islamic State-West Africa Province, the name it adopted in March 2015.

There have been periodic reports of cooperation between Boko Haram and ISIL's Libyan branch. In April, the New York Times cited a U.S. general in reporting that an arms convoy believed bound for Boko Haram from Libya was intercepted in Chad, providing one of the first concrete examples of cooperation.

PROPAGANDA SUPPORT

A U.S. counter-terrorism official, however, said that American intelligence has no evidence to support that report. The region is awash in arms, and it's nearly impossible to determine who is sending what to whom, this official said.

U.S. officials told Reuters that they assess that slicker Boko Haram videos prominently displaying Islamic State logos were produced by ISIL operatives outside the region.

"It was clear to us that there (were) not guys in Nigeria sitting at their laptop putting this stuff together," one official said.A senior U.S. intelligence official said that some Boko Haram fighters have traveled to Libya to "work with Islamic State elements", and that its shadowy leader Abubakr Shekau has established a relationship with the IS Libya branch.

But another U.S. official viewed Shekau's pledge of allegiance to ISIL leader Abu Bakr al-Baghdadi "primarily as a rebranding exercise" aimed at boosting the stature of his group, whose leaders previously said it was aligned with al-Qaeda.

U.S. 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 U.S. 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 neighbors should get U.S. help to crush the group.

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

(Editing by John Walcott and Stuart Grudgings)