Abuja - UN Secretary-General Ban Ki-moon met with President Muhammadu Buhari Monday and promised solidarity in the new Nigerian leader's fight against insecurity, unemployment and corruption."I'm convinced that when you change Nigeria, you will also change Africa," Ban said after meeting Buhari, who has said unchecked corruption could destroy Africa's oil giant.Ban paid tribute to 23 colleagues killed four years ago when a Boko Haram suicide car bomber smashed through the barrier and exploded into the reception area of the UN headquarters in Abuja on August 26, 2011, killing 23 people. He also met with some of the 60 wounded who "suffered terrible injury and trauma," he said."The terrorists attacked the United Nations and destroyed the lives of many colleagues," said Ban. "But we have a mandate to build. To better the lives of people in need. To conquer hunger, disease and illiteracy. To promote respect for people's rights and freedoms."The UN leader called for the unconditional release of 219 abducted schoolgirls held by Boko Haram for nearly 500 days and many other children abducted by the extremist group.His visit comes as a regional army is preparing a long-delayed offensive against Nigeria's Islamic extremists. The 6-year-old uprising has killed 20 000 people, including more than 1 000 this year.The Nigerian air force reported on Monday that a helicopter was hit by extremists during an operation that destroyed two trailers believed to be carrying logistics support for the insurgents near Belel town on the border with Cameroon. A statement said the main rotor blade of the Mi-24V attack helicopter was hit but the pilot managed to fly it back to base.Ban leaves on Monday night for Paris to address a meeting of French ambassadors ahead of a December conference on a new climate agreement.