The US National Security Agency (NSA) collected more than 151 million records of Americans' phone calls last year, even after Congress limited its ability to collect bulk call records.

A report from the office of Director of National Intelligence Dan Coats presented the first measure of the effects of the 2015 USA Freedom Act, which limited the NSA to collecting the phone records and contacts of people that the US and allied intelligence agencies suspect may have ties to "terrorism".

NSA collected the 151 million records even though it had warrants from the secret Foreign Intelligence Surveillance Court to spy on only 42 suspects in 2016, in addition to a handful identified the previous year, the report said.

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Because the 151 million would include multiple calls made to or from the same phone numbers, the number of people whose records were collected would be much smaller, US officials said. They said they had no breakdown of how many individuals' phone records were among those collected.

Politicians have repeatedly asked US intelligence agencies to tell them how many Americans' emails and calls are vacuumed up by warrantless government surveillance programmes.

"This report provides a small window into the government's surveillance activities, but it leaves vital questions unanswered," Senator Ron Wyden said in a statement. "At the top of the list is how many Americans' communications are being swept up."

The NSA has been gathering a vast quantity of telephone "metadata" - records of callers' and recipients' phone numbers and the times and durations of the calls - since the September 11, 2001, attacks.

The spy agency says it doesn't collect the content of the communications.

US officials on Tuesday argued the 151 million records collected last year were tiny compared with the number gathered under procedures that were stopped after former NSA contractor Edward Snowden revealed the surveillance programme in 2013.

A report in 2014 suggested potentially "billions of records per day" were being collected.

"This year's report continues our trajectory toward greater transparency, providing additional statistics beyond what is required by law," said Timothy Barrett, spokesman for the Office of the Director of National Intelligence.

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The new report came amid allegations - recently repeated by US President Donald Trump - that former president Barack Obama ordered warrantless surveillance of his communications, and former national security adviser Susan Rice asked the NSA to "unmask" the names of US people caught in the surveillance.

Both Republican and Democratic members of the congressional intelligence committees have said so far they have found no evidence to support either allegation.

The report said the names of 1,934 "US persons" were "unmasked" last year in response to specific requests, compared with 2,232 in 2015. But it did not identify who requested the names or on what grounds.