A US law prevents the government from selling arms to countries which fail to tackle human rights abuses.
Mr Buhari met President Barack Obama in Washington on Monday to seek further assistance.
Boko Haram has killed some 10,000 people since 2009 and has also kidnapped hundreds of girls and women.
Last month, human rights group Amnesty International said that some 8,000 men and boys had died in Nigerian military custody after being detained as suspected militants.
The military rejected this allegation, Nigeria's president promised an investigation but there have been no further details.
The US has previously promised some $5m (Â£3.2m) in military assistance to the regional coalition helping to fight Boko Haram, which has pledged allegiance to Islamic State.
However, Mr Buhari heavily criticised the US, especially the Leahy Law, which links military sales to human rights.
The Nigerian military did "not possess the appropriate weapons and technology which we could have had if the so-called human rights violations had not been an obstacle," he told the United States Institute of Peace on Wednesday.
"Unwittingly, and I dare say unintentionally, the application of the Leahy Law Amendment by the United States government has aided and abetted the Boko Haram terrorists."
In the latest suspected Boko Haram attack, at least 29 people were killed in bomb blasts at two bus stations in the north-eastern city of Gombe on Wednesday.
At least 11 people were earlier killed in the Cameroonian town of Maroua, just over the border from Gombe.
Boko Haram last year seized a huge area of north-eastern Nigeria, before being beaten back by a regional coalition, including Cameroon, Chad and Niger.
Mr Buhari was elected in March, partly on a pledge to defeat Boko Haram.
Why Boko Haram remains a threat
Who are Boko Haram?

England and Wales Cricket Board (ECB) rules need to be altered to remove the right of all 18 counties to play in the new tournament.
Essex follow Middlesex in their refusal to support the change.
But Durham chairman Sir Ian Botham said his county were fully committed in their support for the new tournament.
He said: "It's financially very sensible to do it and if we prepare properly and come up with the right formula throughout the summer - just look at the success of the Big Bash in Australia and the IPL - then it's proven that the format has great benefit."
Meanwhile, Essex chairman John Faragher said the County Championship, one-day and T20 competitions "must be protected".
The changes require the support of 31 out of 41 of the ECB's voting members.
Faragher continued: "We are focused on expanding cricket in Essex, East Anglia and Metropolitan London, ensuring there are opportunities for all age and ability groups, male and female, to be actively involved in the game.
"We believe that as a result of the proposed changes, these opportunities will be reduced, that our income overall will suffer and the first-class game will be diminished, in contradiction to the ECB's objective which is to grow the game in this country - an objective that is unlikely to be advanced by a competition which would exclude large areas of the country from any involvement in it."
The 41 voting members comprise the 18 first-class counties, 21 recreational boards, the Minor Counties Cricket Association, and MCC - owners of Lord's and therefore Middlesex's landlord.
A number of counties have come out in support of the proposals for the new tournament, with Derbyshire, Leicestershire, Somerset, Sussex and Yorkshire all announcing that they will back the ECB rule change, while others including Glamorgan, Hampshire and Warwickshire have been vocal in their support for a city-based competition.
Kent have asked their members and supporters to give them further feedback before making a decision, while Surrey director of cricket Alec Stewart has expressed concern over the details of the new event.
The US has "aided and abetted" the Boko Haram Islamist militant group by refusing to provide weapons to Nigeria, President Muhammadu Buhari says.
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Essex have become the second county to announce their intention to stand against a new city-based Twenty20 competition, planned for 2020.