The US National Security Agency (NSA) recorded 151 million phone calls made by American citizens last year while tracking several terror suspects, a sobering report by the country’s top intelligence officer has revealed.

The office of Director of National Intelligence Dan Coats broke the news in compliance with the USA Freedom Act, which also limited the agency’s powers to collect phone records and contacts of people.

The law was passed in 2015, after famous whistleblower Edward Snowden revealed the agency’s extensive surveillance programs.

According to the report, the intelligence agency only had obtained enough warrants from the Foreign Intelligence Surveillance Court to spy on 42 terrorism suspects in 2016, but it went ahead with the massive data collection anyway.

The NSA has been collecting phone calls, texts and emails of American people ever since the September 11, 2001 terror attacks in New York.

Although NSA officials say the number is way less than the billions of records that were collected each day, it still signifies the agency’s reach in gathering ordinary people’s personal data.

The NSA's data collection center in Bluffdale, Utah, March 17, 2017. (Photo by AFP)

Under the new regulations, which tend to limit the NSA’s ability in collecting data, the agency can still monitor phone numbers that are “two hops” away from a targeted phone.

This means they would log all the numbers that interacted with the target number and then monitor each of the interacting numbers separately. As a result, the collected data grows in size exponentially.

The report also showed that the US intelligence community had conducted around 30,335 searches specifically looking for Americans by identifying metadata, such as name or phone number, last year. The number stood at around 9,500 searches in 2013.

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The report came days after the NSA admitted to illegally collecting Americans’ emails, an activity it reportedly agreed to stop under a critical 2008 legal authority known as section 702 of the Foreign Intelligence Surveillance Act (FISA).

It was not clear if the NSA had ceased the practice altogether or was simply going to use other similar laws like an executive order by former President Ronald Reagan to keep collecting the data.