The number of phone records collected by the US National Security Agency (NSA) has fallen significantly due to an explosion in mobile-phone use, according to a media report.
The Washington Post, citing current and former US officials, reported that the secretive agency is collecting less than 30% of all of Americans' call records because it cannot keep pace with the growth in mobile-phone use.
In 2006, NSA surveillance covered almost all Americans with mobile phones, but that had declined to less than 30% by last summer, a senior official told the newspaper. The data collected does not include the content of conversations.
The decline reflects Americans' increasing use of mobile phones to replace landlines, as well as the technical challenges facing the NSA, according to the newspaper.
Industry and government figures reveal that the number of active landlines in the country fell 32% to 96 million in 2012, from 141 million in 2008.
In contrast, the number of active mobile phones jumped 28% to 326 million from 255 million. The number of internet-based subscribers doubled to 42 million in 2012 from 21 million in 2008, according to the Federal Communications Commission.
The officials added that the government is taking steps to increase the amount of collection and the NSA is preparing court orders to oblige more telecoms carriers to hand over records.
The NSA's surveillance practices have come under worldwide scrutiny following revelations by whistleblower Edward Snowden, a former contractor at the agency.
Snowden leaked top secret documents to the media, revealing the NSA's bulk collection of phone records and internet activity across the globe.
The NSA claimed the surveillance programme was part of its anti-terror measures, but the Barack Obama administration was forced to introduce reforms following revelations that the agency's targets included prominent figures such as German Chancellor Angela Merkel and Brazilian President Dilma Rousseff.
Obama has asked the US Justice Department and the intelligence community to develop a new system of domestic phone data collection.
They are expected to report back before 28 March.