French Foreign Minister Laurent Fabius has summoned the US ambassador over claims in a newspaper about large scale US spying on French citizens.
The newspaper Le Monde says the data, based on leaks from ex-intelligence analyst Edward Snowden, suggest the National Security Agency (NSA) spied on millions of phone calls and monitored businesses and officials as well as terrorism suspects.
The paper says the NSA spied on 70.3 million phone calls in France in just 30 days between 10 December last year and 8 January 2013.
The agency also apparently captured millions of text messages. The intercepts were apparently triggered by certain key words.
It was unclear whether the content of the calls and messages was stored, or just the metadata - the details of who is speaking to whom - and the paper did not say whether the operation was still in progress.
Prime Minister Jean-Marc Ayrault said he was deeply shocked by the report. "It's incredible that an allied country like the United States at this point goes as far as spying on private communications that have no strategic justification, no justification on the basis of national defence," he told journalists.
The US embassy in Paris categorically denied the US had been involved in any cyber attack on the French government.
The latest revelations follow claims in the German media that US agents hacked into the email account of former Mexican President Felipe Calderon, the BBC reports.
Mr Snowden, a former NSA worker, went public with revelations about US spying operations in June.
Le Monde reported in July that the French government was storing vast amounts of personal data of its citizens on a supercomputer at the headquarters of the DGSE intelligence service.
The paper said connections inside France and between France and other countries were all monitored and emails, text messages, telephone and internet browsing records are stored for years.