The US government threatened to fine Yahoo $250,000 a day if it refused to hand over user data to the National Security Agency, according to court documents unsealed on Thursday.

In a blogpost, the company said the 1,500 pages of once-secret documents shine further light on Yahoo's previously disclosed clash with the NSA over access to its users' data. The size of the daily fine was set to double every week that Yahoo refused to comply, the documents show.

The papers outline Yahoo's secret and ultimately unsuccessful legal battle to resist the government's demands for the tech firm to cooperate with the NSA's controversial Prism surveillance programme, revealed by whistleblower Edward Snowden last year.