'I didn't send classified emails on pvt account'

US presidential candidate Hillary Clinton said on Saturday that she did not use a private email account to send or receive classified information while she was secretary of state, in response to a government inspector's letter this week.

"I did not send nor receive anything that was classified at the time," Clinton said at a campaign stop in Iowa.

The email controversy has dogged Clinton's bid for the presidency, fuelling worries that the frontrunner for the Democratic nomination has tried to sidestep transparency and record-keeping laws.

At least four emails from the private email account that Clinton used while secretary of state contained classified information, Inspector General Charles McCullough, who oversees US intelligence agencies, told members of Congress in a letter on Thursday.

Clinton said on Saturday she had "no idea" what were the emails mentioned in the letter. The information was classified at the time that the emails were sent, McCullough said.

The use of her private email account, linked to a server in her New York home for work, has drawn fire from political opponents since coming to light in March.