Former US attorney general Eric Holder has said whistleblower Edward Snowden performed a "public service" by sparking a debate about secret domestic surveillance programmes, but insisted that he should still return to the US to stand trial.

As a National Security Agency contractor, Mr Snowden leaked details in 2013 of the US government's warrantless surveillance of its citizens before fleeing the country.

He now lives in Russia and faces charges back in the States that could land him in prison for up to 30 years.

Mr Holder said Mr Snowden's leaks had harmed American interests abroad and put intelligence assets at risk.

But Mr Holder added that whatever punishment Mr Snowden receives should be weighed against the "usefulness" of a national debate about the government's secret surveillance operations.

Mr Holder was interviewed by CNN political commentator David Axelrod for a podcast.