WASHINGTON (Reuters) - A United States official said on Thursday that the State Department had been unable to locate all or part of 15 e-mails from Hillary Clinton's personal server that were released this week by a congressional panel probing the 2012 attacks on US facilities in Benghazi, Libya.

"There are... a limited number of instances, 15, in which we could not locate all or part of the content ...," the State Department official said in an e-mail. "The substance of those 15 e-mails is not relevant to the 2012 attacks in Benghazi."

The State Department said it learned the e-mail record was incomplete after US lawmakers investigating the deadly 2012 attack on diplomatic staff in Benghazi, Libya, were given dozens of previously undisclosed e-mails discussing Libya by Sidney Blumenthal, an old friend and informal adviser to Mrs Clinton. Most government workers are required to use government email addresses, and Mrs Clinton's unusual use of a private e-mail address connected to a server in her home while working as the top US diplomat has exposed her to criticism that she side-stepped record-keeping and transparency rules.

Mr Trey Gowdy, the Republican congressman in charge of the select committee investigating the Benghazi attack, said Mrs Clinton's incomplete e-mail record "raises serious questions". "This has implications far beyond Libya, Benghazi and our committee's work," Mr Gowdy said in a statement. "This conclusively shows her email arrangement with herself, which was then vetted by her own lawyers, has resulted in an incomplete public record."

Spokesmen for Mrs Clinton have declined to answer questions about her e-mail records since evidence of the gaps first emerged last week when Mr Gowdy's committee released the additional e-mails provided by Mr Blumenthal.

In March, Mrs Clinton said in an impromptu news conference at the United Nations headquarters that she gave the State Department all her e-mails that "could possibly be work-related".

She said that the 30,490 e-mails she handed over included all that referred to Libya or Benghazi, and included all work-related correspondence from what her office described as "long-time friends".

Most of the 15 previously undisclosed messages between Mr Blumenthal and Mrs Clinton were in the form of long memoranda filled with intelligence on the turmoil in Libya.

Mr Blumenthal could not immediately be reached for comment.