The US National Security Agency (NSA) sometimes uses data it collects for economic purposes, intelligence leaker Edward Snowden reveals in an extract of an interview with a German television chain to be broadcast Sunday.

"If there is information, for example on Siemens, which is in the national interest, but has nothing to do with national security, they will still use this information," said Snowden, according to the German translation of the interview on public television ARD.

The interview was carried out by a journalist for NDR, a regional chain belonging to the broadcaster that has analysed secret documents that Snowden leaked to journalists.

Under top secrecy, the chain this week in Moscow filmed the first interview with Snowden since he left Hong Kong in 2013 to seek refuge in Russia.

The 30-minute interview will be broadcast Sunday at 2200 GMT, with initial extracts to be released during an earlier talk show at 2045 GMT.

On its website, NDR said that Snowden assured he was no longer in possession of any confidential documents, as they had all been handed out to handpicked journalists.

The former NSA contractor said he no longer wants to, or is able to, take part in any future revelations.

Other than the consequences of his revelations about NSA surveillance programs, Snowden will also address "his personal path" to leaking the information.

Snowden, 30, fled the United States in May last year after revealing that his government was collecting telephone data from millions of US citizens, monitoring vast amounts of private Internet traffic and eavesdropping on the conversations of foreign friends and foes alike.

On Thursday, in a question-and-answer session on the "Free Snowden" website, Snowden ruled out returning to the United States, where he said there was no chance of a free trial.

US Attorney General Eric Holder has said he was unlikely to consider clemency for Snowden.