A growing controversy over Democrat Hillary Clinton's use of personal email for work while she was US Secretary of State could drag on for months, threatening to cloud the expected launch of her 2016 presidential campaign.

Clinton tried to cool the brewing firestorm over the emails late on Wednesday, saying she wanted the State Department to release them quickly. But a senior State Department official told Reuters the task would take time.

"The review is likely to take several months given the sheer volume of the document set," the official said.

That will give Clinton's Republican foes plenty of time to take aim at her with allegations that the use of personal email for official duties while secretary of State from 2009 to 2013 was inappropriate.

"I want the public to see my email," Clinton said in a tweet late on Wednesday. "I asked State to release them. They said they will review them for release as soon as possible."

The controversy has landed Clinton in trouble just as she is planning to launch a bid for the Democratic presidential nomination in 2016. It has prompted some Democrats to wonder whether someone else should be their candidate in the bid to succeed President Barack Obama.

A total of 55,000 pages of material covering the time Clinton was in office has been turned over, according to the State Department.

Clinton's tweeted statement came hours after a congressional committee investigating the Sept. 11, 2012, attack on a US diplomatic facility in Benghazi, Libya, issued subpoenas for her emails.