BRUSSELS (Reuters) - The head of the European Union's border agency Frontex, Fabrice Leggeri, has offered to resign, a source told Reuters on Friday, after accusations the body mistreated migrants on the external frontiers of the bloc.

       Frontex spokesman Pior Switalski declined to comment as the management board of the Warsaw-based agency was meeting. Leggeri, who was attending the meeting, was not immediately available for comment.

       The EU's anti-fraud agency OLAF launched an investigation last year into allegations of human rights violations by Frontex, an agency tasked with managing migration on the bloc's external frontiers. OLAF's report has not been made public.

       European Parliament's Erik Marquardt, a German lawmaker with the Greens' faction, said on March 2 that the summary of the report "reveals that Frontex's management was aware of human rights violations and deliberately avoided reporting them."

       (Reporting by Gabriela Baczynska; Editing by Jon Boyle)