Hillary Clinton has denied using 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.

Presidential candidate Hillary Clinton said at a campaign stop in Iowa: “I did not send nor receive anything that was classified at the time.”

The email controversy has dogged Hillary 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 Hillary 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 July 23.

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However, Hillary Clinton said on July 25 she had “no idea” what were the emails mentioned in the letter.

Charles McCullough’s letter said a sampling of 40 of about 30,000 emails sent or received by Hillary Clinton found at least four that contained information the government had classified as secret.

The information was classified at the time that the emails were sent, he 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.

Republicans have accused Hillary Clinton of trying to avoid disclosure laws through her use of private systems.

Hillary Clinton, the frontrunner to represent the Democratic Party in the November 2016 election, has repeatedly said she broke no laws or rules by eschewing a standard government email account.