Hillary Clinton blamed the renewed FBI inquiry into her State Department email system for blunting her momentum in the presidential election and the closure of that inquiry two days before Election Day for energising voters for Donald Trump.

"There are lots of reasons why an election like this is not successful," Clinton told top donors on a farewell conference call on Saturday local time.

"But our analysis is that (FBI Director James B) Comey's letter raising doubts that were groundless, baseless, proven to be, stopped our momentum," she said.

"We dropped, and we had to keep really pushing ahead to regain our advantage - which going into the last weekend, we had. We were once again up in all but two of the battleground states, and we were up considerably in some that we ended up losing. And we were feeling like we had put it back together," she continued.