Washington: US intelligence agencies will stop bulk collection of data documenting calls by US telephone subscribers in June, unless Congress extends a law authorising the spying, US officials said on Monday.

The disclosure that the National Security Agency was collecting metadata generated by domestic telephone users was one of the most controversial revelations made by former NSA contractor Edward Snowden nearly two years ago.

NSA whistleblower Edward Snowden speaking live via telelink at the 2014 SXSW Interactive Festival. Credit:Estibaliz Quezada

A spokesman for President Barack Obama's National Security Council said abandoning the mass collection of domestic telephone data would deprive the country of a "critical national security tool."

The current law, due to expire on June 1, allows the NSA to collect bulk data on numbers called and the time and length of calls, but not their content.