The US National Security Agency collected more than 500 million phone call records of Americans last year, more than triple gathered in 2016, a US intelligence agency report released on Friday said. The sharp increase to 534 million call records from 151 million occurred during the second full year of a new surveillance system established at the spy agency after US lawmakers passed a law in 2015 that sought to limit its ability to collect such records in bulk. The reason for the rapid increase was not clear. Are your laptop, phone camera and microphone spying on you? Why you should cover up your webcam The spike coincided with an increase reported on Friday across other surveillance methods, raising questions from some privacy advocates who are concerned about potential government overreach and intrusion into the lives of US citizens. The tally was still far less than the estimated billions of records collected every day under the NSA’s old bulk surveillance system, which was exposed by former US intelligence contractor Edward Snowden in 2013. The metadata records collected by the NSA include the numbers and time of a call, but not content. Overall increases in surveillance hauls were both mystifying and alarming coming years after Snowden’s leaks, privacy advocates said. “The intelligence community’s transparency has yet to extend to explaining dramatic increases in their collection,” said Robyn Greene, policy counsel at the Washington-based Open Technology Institute that focuses on digital issues. Timothy Barrett, a spokesman at the Office of the Director of National Intelligence which released the annual report, said the government “has not altered the manner in which it uses its authority to obtain call detail records”. The NSA has found that a number of factors may influence the amount of records collected. “These factors include the number of court-approved selection terms – like a phone number – that are used by the target; the way targets use those selection terms; the amount of historical data that providers retain; and the dynamics of the ever-changing telecommunications sector,” Barrett said. “We expect this number to fluctuate from year to year.” Asylum seekers who housed Edward Snowden in Hong Kong claim police illegally targeted them but force denies doing so US intelligence officials have said the number of records collected would include multiple calls made to or from the same phone numbers and involved a level of duplication when obtaining the same record of a call from two different companies. Friday’s report also showed a rise in the number of foreigners living outside the US who were targeted under a warrantless internet surveillance programme, known as Section 702 of the Foreign Intelligence Surveillance Act, that Congress renewed earlier this year. That figure increased to 129,080 in 2017 from 106,469 in 2016, the report said, and is up from 89,138 targets in 2013, or a cumulative rise over five years of about 45 per cent. US intelligence agencies claim Section 702 a vital tool to protect national security but privacy advocates say the programme incidentally collects an unknown number of communications belonging to Americans.