Hillary Clinton has said she wished she had made a "different choice" and not used a private email account while serving as US secretary of state.

"I'm sorry this has been confusing," she told the cable news channel MSNBC.

Her use of private email has generated a barrage of criticism as Clinton runs for the Democratic presidential nomination for the 2016 election.

Critics say that her set-up was not secure, contrary to government policy, and meant to shield her from oversight.

Political analysts - including fellow Democrats - have said the Clinton campaign has stumbled in its response to the controversy and Clinton had not seemed contrite - at times even making jokes about the email issue.

A more somber, Clinton took full responsibility in Friday's interview, saying she didn't "stop and think" about how use of a private email account would be perceived.

It has been a major issue in the presidential race. Polls show an increasing number of voters view her as "untrustworthy" due in part to the questions surrounding her email use.

Under US federal law, officials' correspondence is considered to be US government property.

The emails often discussed mundane or non-urgent matters

Government employees are encouraged to use official email accounts although some top officials have used personal accounts in the past.

In March, Clinton said she and her lawyers made the decision over what would be considered work-related email when the state department asked for records from former secretaries of state.

The emails deemed work-related were about half of the 60,000 emails she sent in total during her time in office. The emails she deemed personal were deleted, Clinton said.

Since then, the state department has been releasing the emails to the public in batches about once a month.