Hillary Clinton takes part in a roundtable of young Nevadans discussing immigration as she campaigns for the 2016 Democratic presidential nomination at Rancho High School in Las Vegas, May 5, 2015. REUTERS/Mike Blake

(Reuters) - The U.S. State Department has deemed it "not acceptable" for staff to conduct official business on a private email server as presidential contender Hillary Clinton did when she was secretary of state, a senior department official told senators on Wednesday.

Testifying before the U.S. Senate Judiciary Committee, Joyce Barr, the State Department's assistant secretary of administration, said that, in the course of recovering Clinton's emails, the agency had clarified its guidelines on record keeping.

Asked by Republican Senator Thom Tillis if the practices used by Clinton were still acceptable at the department, Barr said, "We continue to do training but we've sent department notices, telegrams, we've talked to directors. I think the message is loud and clear that that is not acceptable."

Asked to clarify whether she meant it was both unacceptable now and should have been unacceptable in the past, Barr said, "Going forward, yes."

Clinton is the leading contender for the Democratic Party's presidential nomination in 2016. She is expected to testify soon before a U.S. House of Representatives panel investigating the deadly 2012 attacks on U.S. diplomatic facilities in Benghazi, Libya, and to answer questions about her email practices.

Clinton’s exclusive use of a personal email account for government business as secretary of state emerged in a New York Times report in March that prompted concerns about transparency and security.

Clinton explained that she thought it would be simpler to just have one account so she would only need to carry one email handheld device. She has also said that while she should have used a separate government email account, she violated no rules.

Barr told senators on the Republican-controlled committee on Wednesday that the State Department now has all of the official emails that Clinton had turned over to the department but was relying on her word that nothing official was withheld.

Under questioning from Republican Senator John Cornyn, she said she had not known that Clinton was operating exclusively on a private email server while Clinton was secretary of state, nor was Barr aware of anyone else in the government who was now operating that way.

Senator Tillis said it was a "bad decision" on Clinton's part to use a private server and for the State Department to allow it.

"I hope that we go so far as to say if you do this in the future, you get fired," said the freshman senator.