Nigeria's air force said it had killed a number of senior Boko Haram fighters and possibly their overall leader, and U.S. Secretary of State John Kerry arrived for talks on tackling the militants.

Government planes attacked the Islamist group inside the Sambisa forest in its northeast heartland on Friday, the air force said, adding that it had only just confirmed details of the impact of the raid.

"Their leader, so called 'Abubakar Shekau', is believed to be fatally wounded on his shoulders," the statement by military spokesman Colonel Sani Kukasheka Usman added, without going into details on the source of its information.

Kerry did not make a direct reference to the reported air raid on his arrival on Tuesday, but his administration has paid close attention to the fight against a militant group that has declared allegiance to Islamic State and destabilized a whole region by attacking Nigeria's neighbors.

On his first stop in the northern city of Sokoto, the top U.S. diplomat said the struggle against Boko Haram would only succeed if it tackled the reasons why people join militant groups and gained the public's trust.

"It is understandable that, in the wake of terrorist activity, some are tempted to crack down on anyone and everyone who could theoretically pose some sort of threat. But extremism can't be defeated through repression or fear," he said.

U.S. PLANES

Nigeria has been pushing the United States to sell it aircraft to take on Boko Haram - a group that emerged in northeast Borno region seven years ago. The militants have killed an estimated 15,000 people in their fight to set up an Islamist state.

Under Nigeria's last president, Goodluck Jonathan, the United States had blocked arms sales and ended training of Nigerian troops partly over human rights concerns such as treatment of captured insurgents.

But the new administration of President Muhammadu Buhari has argued its human rights record has improved significantly enough to lift the blockade.

In May, U.S. officials told Reuters that Washington wanted to sell up to 12 A-29 Super Tucano light attack aircraft to Nigeria in recognition of Buhari's reform of the country's army. Congress needs to approve the deal.

Kerry said Buhari had made "a strong start at all levels of government" since taking office in May 2015, without referring specifically to rights abuses.

Kerry was due to visit Buhari later in the capital Abuja, officials said.

There was no immediate reaction from Boko Haram, which communicates with the media only by videos. The military has reported the death of Boko Haram's Shekau in the past, only to have a man purporting to be him appear later, apparently unharmed, making video statements.

There have been recent signs of rifts between at least parts of Boko Haram and Islamic State. The global militant organization announced a new leader for what it described as its West African operations this month - an account that Abubakar Shekau appeared to contradict in a later video message.