In a conference call around midnight Monday, Defense Secretary Jim Mattis will officially resign and turn over his authorities to Deputy Defense Secretary Patrick Shanahan, who will serve as acting secretary of defense until President Donald Trump settles on a permanent replacement, a defense official said Friday.

There had been confusion at the Pentagon throughout this week on the actual process for the transition, since acting secretaries are not required to be sworn in. The defense official said the phone call would be the vehicle for the transfer of power.

"The position of the secretary of defense is a command position, and the purpose of the call is to ensure all members within the government know who is in command," the defense official said in a statement.

Contact will be made by the communications team in the Office of the Secretary of Defense, which will then bring Mattis, a retired Marine general, and Shanahan, a former Boeing Co. executive, into the call.

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"The call will be complete by 11:59 p.m. on December 31st," the official said, adding that it will follow an approved script, which could not be released.

"Once the phone call is complete, the Department of Defense will notify the White House that Mr. Shanahan has assumed the authority of the acting secretary of defense," the official said.

Mattis issued a letter of resignation last Thursday, a day after Trump made the surprise announcement that he was ordering the withdrawal of the estimated 2,000 U.S. troops in Syria.

In his letter, Mattis reaffirmed his commitment to working through alliances and partnered local forces, adding that Trump deserves a defense secretary whose beliefs are more aligned with his own.

Mattis offered to stay on the job until the end of February to give Trump time to consider a replacement, but the president ordered him out by Jan. 1.

Mattis has been at the Pentagon throughout the week but has issued no statements for the record. Shanahan was also in the building Friday.

In a series of tweets, Trump has praised Shanahan's work on finances at the Pentagon, but it was unclear whether he would be considered as a permanent replacement.

Before announcing in late 2016 that he would nominate Mattis upon taking office, Trump also considered former Sen. Jim Talent, R-Missouri; retired Army Lt. Gen. Keith Kellogg; retired Army Gen. Jack Keane; and others to lead the Pentagon.

-- Richard Sisk can be reached at Richard.Sisk@Military.com.