The White House says the chairman of the federal government's board overseeing surveillance and civil liberties has resigned from his post two years before the end of his term.

David Medine, the departing head of the Privacy and Civil Liberties Oversight Board, led the agency in its critical 2014 report on warrantless mass surveillance program run by the National Security Agency.

The board's report found the program had a flimsy legal foundation and did not lead to significant counterterrorism investigations. President Barack Obama commissioned the review after surveillance revelations made by Edward Snowden.

In a statement, President Barack Obama praised Medine for "skilled stewardship" during the administration's reassessment of its surveillance operations. In a separate statement Medine said he would advise on privacy for a development organization.