The US National Security Agency (NSA) infected 50,000 networks with malware, Dutch newspaper NRC has reported.

The Tailored Access Operations department used it to steal sensitive information, according to a censored slide leaked by former NSA contractor Edward Snowden.

NRC said 20,000 networks had been hit in 2008, with the program recently expanded to include others in Rome, Berlin, Pristina, Kinshasa, Rangoon.

The NSA declined to comment.

The malware could be put in a "sleeper" mode and activated with a click of a button, the paper said.