Democratic US presidential candidate Hillary Clinton said on Saturday she and her family paid a State Department staffer with personal funds to maintain a private email server that she used for both personal and government matters while secretary of state.

After a campaign appearance in New Hampshire, Clinton told reporters the payments went to information technology specialist Bryan Pagliano, who this week declined to produce documents and testify before a US House of Representatives committee about the server, invoking his constitutional right against self-incrimination.

"With respect to personal services that he provided to me and my family, we obviously paid for those services and did so because, during a period of time, we continued to need his technical assistance. And I think that's in the public record," Clinton said.

Clinton, a former US senator and first lady who is the front-runner for her party's 2016 presidential nomination, has been criticized for using the unsecured server to conduct government business when she was the top US diplomat from 2009-13 as well as for how she handled classified information.

Campaign spokesman Nick Merrill said on Twitter: "Bryan was hired by the Clinton family as a consultant in order to help out periodically with the management of the system in Chappaqua that hosted the family's emails."

Chappaqua is the New York town where Clinton lives.

The Washington Post earlier reported the Clintons paid Pagliano $5,000 for computer services before he joined the State Department, citing an April 2009 financial disclosure form he filed.

Even after he arrived at the State Department in May 2009, the Clintons continued to pay Pagliano to maintain the server, the Post reported. The paper quoted a campaign official as saying the arrangement with Pagliano ensured that taxpayer dollars were not spent on a private server that was also used by Clinton's family and aides to former President Bill Clinton.

Pagliano was the IT director for Clinton's unsuccessful 2008 presidential campaign and went to work for the State Department when Clinton took up the Cabinet post in the Obama administration.

Clinton has in the past hired staff to work for her simultaneously in public and private capacities, most notably top aide Huma Abedin, the Post said.

Clinton said on Friday she was sorry that her use of a personal email account while secretary of state had caused confusion, and blamed herself for "not thinking a lot" about the matter when she took the job.