Nigerian presidential challenger Muhammadu Buhari recorded thumping majorities in key northern states on Monday, as the United States and Britain expressed concerns about meddling with the vote count. Buhari, a 72-year-old former military ruler who has campaigned as a born-again democrat intent on cleaning up the corrupt politics of Africa's most populous nation, won 1.1 million votes in the flashpoint city of Kaduna. President Goodluck Jonathan, a 57-year-old southern Christian, won 484,000 votes there.