Asked what the AU was doing to help, Mull Katende said it was engaged in "quiet diplomacy, because some of this information, we don't want it to reach those who abducted the children".
"We look forward to their recovery as soon as possible, and all heads of state of the African Union have offered their support," he said after talks between AU and UN counterparts in New York.
The Ugandan diplomat said Nigeria could learn a lesson from his own country's 20-year war against Lords Resistance Army rebels, "on how to isolate Boko Haram from the population".
Asked how negotiations for the girls' release could be advanced, Katende told reporters: "There are intensified efforts and a number of countries are helping Nigeria."
"Now some of those efforts cannot come into the public, but these efforts are there, and we are hoping that these girls can be recovered very soon," he added.
He said the African Union wanted to strengthen its institutions and intelligence cells devoted to terrorism and was "employing a wide range of measures," without going into specifics.
Nigeria's response to the mass April 14 abduction has been widely criticised, and the crisis has brought unprecedented international attention to Boko Haram's extremist uprising.
Washington recently began surveillance drone flights over northeastern Nigeria to try to track down the location of the kidnapped girls.
France also has forces in Chad, and Britain and the United States have sent small teams of specialists to Nigeria to assist President Goodluck Jonathan's government in the search. – AFP, June 7, 2014.