U.S. Senator Feinstein is trailed by reporters as she walks to the weekly Democratic caucus policy luncheon at the U.S. Capitol in Washington Thomson Reuters

The top Democrats on the US Senate and House of Representatives intelligence committees called on Wednesday for the White House to consider declassifying and releasing any intelligence community assessments of the hack into the Democratic National Committee email system.

Experts have attributed the breach to Russian hackers.

"Given the grave nature of this breach and the fact that it may ultimately be found to be a state-sponsored attempt to manipulate our presidential election, we believe a heightened measure of transparency is warranted," Senator Dianne Feinstein and Representative Adam Schiff wrote in a letter to President Barack Obama.

Obama said in an interview broadcast by NBC News on Tuesday that the Federal Bureau of Investigation was investigating the leak of more than 19,000 DNC emails, which showed the committee had favored Hillary Clinton over Bernie Sanders for the party's presidential nomination.