Asked about setting up the private email account by ABC News, Clinton said: “That was a mistake. I’m sorry about that. I take responsibility, and I’m trying to be as transparent as I possibly can.”
The frontrunner for the Democratic presidential nomination did not apologise for using a private email system when asked directly by NBC News on Friday, saying she was “sorry that this has been confusing to people”.
In an interview with The Associated Press on Monday, she said an apology was not necessary because what she did was “allowed” by the US State Department.
The issue has dogged Clinton’s presidential campaign for months. Despite a big fundraising advantage and a slew of endorsements from party leaders, her standing with voters has slipped, multiple polls show a majority of Americans do not find her honest and trustworthy.
Clinton’s advisers say she’ll more fully address the email saga as the campaign presses on.
Top campaign officials have started emailing memos to anxious supporters and convening late-night conference calls with prominent Democrats.
Republican presidential candidates see the issue as a way to discredit Clinton.
And Republicans in Congress are using questions about whether she withheld pertinent emails to bolster a congressional investigation into the fatal 2012 terror attacks in Benghazi, Libya.
Two powerful Senate chairmen said they are considering seeking one or more immunity orders to compel testimony from Bryan Pagliano, a former aide to Clinton whose lawyers have indicated he will refuse to answer questions under his constitutional right to protect himself against prosecution.
Mr Pagliano was paid to maintain Mrs Clinton’s personal server while she was secretary of state.
Senate Judiciary Committee Chairman Chuck Grassley and Homeland Security and Government Affairs Committee Chairman Ron Johnson, both Republicans, wrote to Mr Pagliano that they are considering seeking the immunity in an effort to compel him to testify, while protecting him from any prosecution.
Clinton told ABC News she feels it was a mistake to use her private email account and acknowledged that she could have handled the resulting inquiries more effectively.
“I do think I could have and should have done a better job answering questions earlier. I really didn’t perhaps appreciate the need to do that,” she said.
She reiterated her apology in a late-night note to voters on Facebook.
Clinton’s support in early voting states has declined against rival, Vermont Senator Bernie Sanders, as she has sought to address questions about her use of a private email server while serving as President Barack Obama’s secretary of state.
She turned over about 55,000 pages of emails to the State Department that she sent and received using a server set up at her New York home.
Clinton said it would have been a “better choice” for her to use separate email accounts for her personal and public business during her time at the State Department.