WASHINGTON (Reuters) - The United States Senate blocked legislation to extend for two months a programme in which spy agencies sweep up vast amounts of data about Americans' telephone calls early on Saturday, adding to uncertainty about the future of the programme.

The Senate voted 45-54 on the procedural measure to stop debate on the extension of provisions of the "USA Patriot Act", falling well short of the 60 votes in favour needed to advance the measure.

The legal authorities that allow the collection of so-called telephone "metadata" expire on June 1.

Republican US Senate Majority Leader Mitch McConnell, speaking shortly after the block, said the Senate would return to Washington on May 31 to consider ways to prevent the expiration of the programme on June 1.

The Senate will return from its Memorial Day holiday recess a day early.