It is being reported that the National Security Agency is gathering nearly five billion records a day on the whereabouts of mobile telephones around the world.
That is according to documents leaked by whistleblower Edward Snowden published by the Washington Post.
The newspaper says the NSA inadvertently gathers US location records, along with the billions of other records it collects by tapping into worldwide mobile network cables.
The programme is detailed in documents given to the Post by former NSA systems analyst Edward Snowden.
Such data means the NSA can track the movements of almost any mobile phone around the world, in addition to tracking who that cell user is calling.
The report said the NSA does not target Americans' location data intentionally, but acquires a substantial amount of information on the whereabouts of domestic cellular telephones "incidentally."
One manager told the newspaper the NSA obtained "vast volumes" of location data by tapping into the cables that connect mobile networks globally and that serve US mobile phone as well as foreign ones.
A spokesman for the Office of the Director of National Intelligence declined to comment on the report.
NSA officials have said no NSA programme gathers data on US mobile phones inside the US.
'Guilt by association'
Senior Staff Attorney at the non profit advocacy Electronic Frontier Foundation Kurt Op-Sal told Al Jazeera that the programme was against civil liberties.
"It is being used to track people to find out associations between one person and another and to build guilt by association," he said.
"If we want a future that has freedom we need to have privacy.
"The UN has recognised a freedom of association without having the government making assumptions based on those associations.This programme is designed to destroy that."
Facing a public outcry and concern that programmes are targeting average Americans as well as international terrorism suspects, Republican and Democratic members of Congress are writing legislation to clamp down on the data collection and increase public access to information about it.
Advocates responded to the Post report by calling on Congress to take up legislation to reform NSA data-gathering programmes.
"How many revelations of NSA surveillance will it take for Congress to act? Today's news is the latest startling blow to the right to privacy," Zeke Johnson, director of Amnesty International USA's Security and Human Rights, said in a statement.