The Obama administration has provided details about its plan to end its bulk collection of millions of records of phone calls made in the US, and introduce new procedures required to get permission from a judge before asking phone companies for data.
The gathering of the information, known as metadata, triggered a national debate over privacy rights when the extent of the surveillance programme was exposed last year by former National Security Agency (NSA) contractor Edward Snowden, who is currently in Russia under temporary asylum.
Under the administration's plan, instead of telephone metadata being collected and stored in bulk from telephone companies by the NSA, companies themselves would hold the data and be required to respond to specific, court-approved queries about it from the NSA.
Phone companies would have to provide data from their records quickly and in a usable format when requested by the government, a senior administration official told reporters on condition of anonymity.
The plan would also allow the government to seek such data without a court order in a national security emergency.
But some US officials who examined the president's proposals said they left important issues unresolved, the Reuters news agency reported.
Officials familiar with current laws and regulations governing how telephone companies handle such data said that Obama's plan raises, but does not answer, significant practical questions about how companies would collect and store such data.
The White House proposal would end the practice of holding onto records for five years so the numbers can be searched for national security purposes.
Instead, the White House is expected to propose that the phone records be kept for 18 months, as the phone companies are already required to do by federal regulation.
Obama, who has been meeting world leaders in Europe, has been grappling with a backlash to US government surveillance programmes since classified details about the extent of data-gathering were first leaked by Snowden.
The president has defended use of the data to protect Americans from attacks.
His plan seeks to hold on to "as many capabilities of the program as possible" while ending the government's role in controlling the database, the New York Times quoted an Obama administration official as saying last week.