LAGOS (Reuters) - Nigeria's Access Bank has obtained shareholders' approval to raise up to 100 billion naira ($504 million) from the debt market either through a public offering or a private placement, the lender said on Thursday. Access Bank spokesperson Busola Osilaja said the bank had sought the approval as part of requirements to be able to raise funds at an opportune time. She said shareholders' voted in favour of the plans at a meeting on Wednesday. Last year the top tier commercial lender raised 42 billion naira from existing shareholders to fund expansion and lending, less than the 53 billion naira it had planned due to weak equity markets. ($1 = 198.55 naira) (Reporting by Oludare Mayowa; editing by Chijioke Ohuocha)