WASHINGTON - The US National Security Agency collected more than 151 million records of US citizens' phone calls last year, even after Congress limited its ability to collect bulk phone records, according to an annual report issued on Tuesday by the top US intelligence officer.

The report from the office of Dan Coats, director of national intelligence, was the first measure of the effects of the 2015 USA Freedom Act, which limited the NSA to collecting phone records and contacts of people who US and allied intelligence agencies suspect may have ties to terrorism.

It found that the agency collected the 151 million records even though it had warrants from the secret Foreign Intelligence Surveillance court to spy on only 42 terrorism suspects in 2016, in addition to a handful identified the previous year.

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 - but not their content - since the Sept 11, 2001, attacks.

The report came as Congress faced a decision on whether to reauthorize Section 702 of the Foreign Intelligence Surveillance Act, which permits the NSA to collect foreign intelligence information on non-US persons outside the United States, and is scheduled to expire at the end of this year.

Privacy advocates have argued that Section 702 permits the NSA to spy on internet and telephone communications of US citizens without warrants from the secret Foreign Intelligence Surveillance Court, and that foreign intelligence could be used for domestic law enforcement purposes in a way that evades traditional legal requirements.

The report did not address how frequently the FBI obtained information about US citizens while investigating a foreign intelligence matter, however.

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

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

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

Because the 151 million would include multiple calls made to or from the same phone numbers, the number of people whose records were collected also would be much smaller, the officials said.