Former US Secretary of State Hillary Clinton was called on 23 April to testify at a public hearing next month by the chairman of a House committee investigating the 2012 attacks in Benghazi, Libya.

The Democratic presidential candidate is expected to be asked about her use of a private email account and server during her time as secretary of state, the Associated Press reported. Clinton is being asked to testify during the weeks of 18 May and 18 June.

In a statement, Republican Representative Trey Gowdy of South Carolina said, "With her co-operation and that of the State Department and [Obama] administration, Secretary Clinton could be done with the Benghazi Committee before the Fourth of July."

According to the Huffington Post, Gowdy initially requested Clinton appear for a private interview with the committee. However, Clinton's attorney David Kendall replied in a letter on 22 April that the former senator would only agree to testify in public.

"In November, I informed your staff of Secretary Clinton's willingness to testify publicly before the Select Committee on Benghazi. In the succeeding months, she has reiterated her willingness and I do so again on her behalf," Kendall wrote. "There is no reason to delay her appearance or to have her testify in a private interview."

The Hill reported that Gowdy's staff signalled that the committee's investigation could continue on until 2016. Democrats on the committee said Gowdy was moving the committee's work too slowly and was dragging the investigation to hurt Clinton's presidential campaign.

Maryland Representative Elijah Cummings and California Representative Adam Schiff released a statement saying Gowdy was moving the investigation at a "glacial pace," the Huffington Post reported.

Republicans' 'conspiracy theories'

"The Republicans's multi-year search for evidence to back up their Benghazi conspiracy theories has turned up nothing," Cummings said. "The Select Committee has identified no evidence — documentary, testimonial, or otherwise — to support claims that Secretary Clinton ordered a stand-down, approved an illicit weapons programme, or any other wild allegation Republicans have made for years."

He continued: "Instead, this appears to be a coordinated attempt by Republicans to drag out this taxpayer-funded search for anything they can use against Hillary Clinton, while their political arm raises campaign funds off the deaths of four Americans."