But the official said Washington is committed to helping fight the Islamist militants and rescue over 200 girls seized from their school a month ago.
U.S. officials have said the effort to retrieve the girls is now a top priority but has been complicated by Nigeria's early reluctance to accept assistance, and U.S. rules banning aid to foreign forces that have committed human rights abuses.
"In general Nigeria has failed to mount an effective campaign against Boko Haram," Alice Friend, the Pentagon's principal director for African Affairs, told a hearing of the Senate Foreign Relations Committee's Africa subcommittee.
"The Department has been deeply concerned for some time by how much the Government of Nigeria has struggled to keep pace with Boko Haram's growing capabilities," Friend said.
Friend said it was "troubling" that atrocities have been perpetrated by some Nigerian forces during operations against Boko Haram.
Robert Jackson, acting assistant Secretary of State for African Affairs, said Washington has urged Nigeria to reform its approach to the group. "When soldiers destroy towns, kill civilians and detain innocent people with impunity, mistrust takes root," he said.
The U.S. embassy in Abuja offered help almost immediately after the kidnapping. But it was two weeks before U.S. Secretary of State John Kerry called Nigerian President Goodluck Jonathan to offer aid, which was accepted on May 4, Jackson said. Friend said U.S. reconnaissance flights started days later.
Nigeria has been reluctant to designate Boko Haram as a terrorist threat at the United Nations, but Jackson said it has changed its position and he expected that designation "imminently."
The U.S. officials said Boko Haram is a regional threat that is becoming international, with ties to al Qaeda. They said the Pentagon and Department of State were developing a "regional response," including improved security along Nigeria's borders with Chad, Niger and Cameroon.
"We've definitely determined that there are links between al Qaeda in the Islamic Mahgreb and Boko Haram. They have probably provided at least training, perhaps financial support," Jackson said.