A senior Democratic senator on Thursday said he intended to investigate Yahoo's cyber security practices after the company announced it had uncovered another massive data breach dating to August 2013 that affected more than 1 billion user accounts.

The haul of data is the largest on record and appears to be separate from a 2014 attack the beleaguered internet company disclosed earlier, Yahoo said Wednesday.

"This most recent revelation warrants a separate follow-up and I plan to press the company on why its cyber defences have been so weak as to have compromised over a billion users," Senator Mark Warner of Virginia said in a statement to Reuters.

Warner, who will become the top Democrat on the Senate Intelligence Committee next year, described the hacks as "deeply troubling." He said he had repeatedly asked Yahoo for briefings about the 2014 hack, which affected 500 million accounts, but had not received a response.

-Reuters