The US National Security Agency has been hacking data links connecting Yahoo and Googles data centres, according to leaks by Edward Snowden.
Millions of records were gleaned daily from the internet giants internal networks, documents published by the Washington Post indicate.
The agencys director said it had not had access to the companies computers.
Gen Keith Alexander told the media that they are not authorised to go into a US companys servers and take data."
But correspondents say this is not a direct denial of the latest claims.