The US National Security Agency is able to collect only a portion of the country's telephone data because of limited capacity to gather and store it, according to a newspaper report.
The NSA can collect only 20 to 30 per cent of US telephone activity, The Washington Post reported on Friday.
The explosive growth in mobile phone use is driving the agency's capacity problem, government officials told the paper, speaking on condition of anonymity.
The agency is working to create databanks to keep up with the daily deluge of electronic information.
In 2006, the NSA had the ability to capture and store almost all telephonic data generated in the United States, the officials said, and the intelligence agency was trying regain that level.
The NSA has been under fire since its bulk collection program was first revealed in June by former NSA contractor Edward Snowden.
On January 17, President Barack Obama announced changes to the NSA's bulk collection of telephone metadata.
While Obama did not call for the end of bulk data collection, his changes included allowing the NSA to query specific data only when a court had found reasonable suspicion of a connection to a terrorist organisation.
Obama said further queries could extend only two steps beyond the original source, instead of three.
Reforms in the program, however, have not satisfied all interested parties.
Twitter on Friday became the most recent internet company to complain, saying it was pressing the US Justice Department for more transparency on surveillance.
The social media website wants to be able to provide the public greater transparency about the requests for information it receives for national security purposes.
"In light of ongoing revelations about government surveillance, we've taken a public stand in support of increased transparency and global government surveillance reform," the company said in a blog post.
Twitter noted that the US Department of Justice reached an agreement last week allowing limited disclosure of national security requests.
While Twitter said that was a step in the right direction, it did not "provide meaningful or sufficient transparency for the public".
