The Democratic presidential nominee hopeful has faced intense scrutiny after it was revealed that she used a private email server when she was the US secretary of state.
She has been forced to hand over her server containing 30,000 emails from her time in the State Department, which she headed from 2009 until 2013, to an internal government watchdog.
Only about 40 emails have been analysed so far, but two of them contain details of US drone strikes, potentially breaking laws governing the handling of top secret intelligence, AP reported.
One of the emails contain discussions of a news article on US drone strikes in Pakistan and other countries, and the other had information that originally came from the CIA.
Clinton did not personally send the emails and they do not contain any direct references to any critical intelligence strategies or practices, the officials told the AP.
The news threatens to derail her presidential bid, with former vice president Al Gore – who served under her husband Bill Clinton – being the latest heavyweight Democrat name to be reportedly considering competing for the nomination.
Clinton, who has been slow to confront the controversy head-on, came out fighting during a speech in the all-important swing state of Iowa, according to the Washington Post.
"It's not about Benghazi," Clinton said. "And you know what, it's not about e-mails or servers either. It's about politics."
"I won't get down in the mud with them," she added, vowing to fight back against the controversy. "I won't play politics with national security or dishonour the memory of those we lost. I won't pretend this is anything other than what it is: the same old partisan games we've seen so many times before. I don't care how many super PACs and Republicans pile on. I've been fighting for families and underdogs my entire life and I'm not going to stop now."
Democrat strategists are concerned that the controversy surrounding Clinton's emails is hurting her public image after she was forced to hand over the private email server to the FBI and the Department of Justice is continuing to conduct an investigation into whether she compromised state secrets.
A recent Fox News poll found that 58% of American potential voters think Clinton lied about the classified information in her emails, compared with 33% who think there is another explanation, according to Politico.