ABUJA, May 18 (Reuters) - Nigeria’s President Muhammadu Buhari requested on Tuesday parliamentary approval for new external borrowing of $6.18 billion. Nigeria will seek to raise $3 billion or more in Eurobonds as part of the new debt, the president wrote in a letter to parliament detailing the borrowing plan. (Reporting by Camillus Eboh and Chijioke Ohuocha; Writijng by Paul Carsten; Editing by Catherine Evans) Our Standards: The Thomson Reuters Trust Principles.