Nigeria's President Muhammadu Buhari warned Washington on Wednesday that a US refusal to arm his troops because of "so-called human rights violations" only helps Boko Haram.

The 72-year-old former general has been warmly received in the US capital on his first visit since his March election raised hopes of reform in Africa's troubled giant.

But he departs with little practical military assistance in his battle against the Islamist militants who have turned the northeast of his country into a bloody war zone.

The US government has vowed to help Nigeria defeat the insurgency but it is prohibited under law from sending weapons to countries that fail to tackle human rights abuses.

"Regretably, the blanket application of the Leahy Law by the United States on the grounds of unproven allegations of human rights violations levelled against our forces has denied us access to appropriate strategic weapons to prosecute the war," he said.

Addressing an audience of policy-makers, activists and academics in Washington, Buhari complained that Nigerian forces had been left "largely impotent" in the face of Boko Haram's campaign of kidnapping and bombings.

"They do 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 said.

"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."

He appealed to both the White House and the US Congress to find a way around the law -- introduced by Senator Patrick Leahy in 1997 -- and to supply his troops with high-tech weapons under a deal "with minimal strings."

Buhari, who ruled Nigeria as a military strongman between 1983 and 1985, returned to office in March as the country's first opposition challenger to defeat an incumbent in a largely fair poll.

His victory triggered a wave of optimism for oil-rich Nigeria, which has Africa's biggest population and economy but many deep and seemingly intractable problems.

Since 2009, Boko Haram has been trying to establish an Islamist breakaway state in a conflict that has seen 15,000 people killed and 1.5 million displaced.

The group's brutality and in particular the mass kidnapping and enslavement of schoolgirls has shocked world opinion, but Nigeria's own security forces also face criticism.

In June, rights watchdog Amnesty International said there is sufficient evidence to launch an investigation into senior Nigerian officers for war crimes.

In a detailed 133-page report, the group blamed the army for the extrajudicial execution of 1,200 people and the torture or arbitrary detention of thousands more.

Buhari insists that the charges are not proven, but he has replaced his senior military commanders and has promised to investigate the allegations.