Senator John McCain (R-AZ) speaks to reporters as he arrives for a vote on Capitol Hill in Washington, U.S., October 18, 2017. REUTERS/Joshua Roberts

WASHINGTON (Reuters) - Senator John McCain suggested on Thursday that the Senate Armed Services Committee, the powerful congressional panel he chairs, may consider issuing a subpoena to make the White House’s top cyber security official testify.

White House cyber security coordinator Rob Joyce did not appear before the committee on Thursday, as requested, to discuss cyber threats facing the United States. His vacant seat drew frustration from lawmakers in both parties.

Senator Bill Nelson, a Democrat on the panel, urged McCain to consider a subpoena to compel Joyce to appear.

“I think that needs to be discussed in committee,” McCain said.

In a statement, a White House spokesman said it has been “longstanding practice” of presidents of both parties to not make White House advisers available for congressional testimony.

“This practice is rooted in the separation of powers and in the confidentiality interests of the executive branch,” the spokesman said. “Officials from relevant departments and agencies are available to accommodate the committee’s legitimate oversight needs without violating the confidentiality interests that attach to White House staff.”

McCain countered, however, that the absence was a “misinterpretation” of the president’s executive authority to have private counsel.

Several other senators expressed frustration at Joyce’s absence.

“The empty chair is outrageous,” Democratic Senator Claire McCaskill said. “I am disgusted there is not a representative here.”