Visiting U.S. Secretary of Defense Mark Esper speaks during a joint press conference in Kabul, capital of Afghanistan, on February 29 (XINHUA)

U.S. Secretary of Defense Mark Esper said on June 3 that he opposed the use of active-duty military forces to quell protesters across the country demonstrating against police brutality and racial injustice.

"I say this not only as Secretary of Defense, but also as a former soldier, and a former member of the National Guard: The option to use active-duty forces in a law enforcement role should only be used as a matter of last resort, and only in the most urgent and dire of situations. We are not in one of those situations now," the Pentagon chief said at a press conference.

Esper's statement put him at odds with U.S. President Donald Trump, who on June 1 said he might invoke the Insurrection Act of 1807 to crack down on the nationwide unrest over the killing of George Floyd, an unarmed black man in Minneapolis who was suffocated to death on May 25 as a white police officer kept kneeling on his neck for almost nine minutes.

"I do not support invoking the Insurrection Act," Esper said. The 213-year-old law authorizes the president to unilaterally deploy military forces on domestic soil for law enforcement purposes.

Responding to a question about whether Trump has lost confidence in Esper following the latter's remarks, White House Press Secretary Kayleigh McEnany said she was not aware of Trump's opinion yet. "With regard to whether the president has confidence, I would say if he loses his confidence in Secretary Esper, I'm sure you all will be the first to know," she told the reporters.

McEnany also said that the president "has the sole authority" to invoke the Insurrection Act, and that "if needed, he will use it."

Washington, D.C. Mayor Muriel Bowser, for her part, said on June 3 that the district was "examining every legal question about the president's authority to send troops, even national guard, to the District of Columbia, and if he has to make any other legal steps to do that."

Trump claimed in a speech on June 1 at the White House Rose Garden that he would send in "thousands and thousands of heavily armed soldiers, military personnel and law enforcement officers" to restore order should state and local officials fail to do so.

After finishing the speech, Trump walked to the historic St. John's Episcopal Church where he held a Bible in his hand and later posed for a photo op flanked by senior administration officials, including Esper.

Police reportedly used tear gas, flash grenades and rubber bullets against largely peaceful protesters to clear the way for the president and his entourage. McEnany, however, said neither tear gas nor rubber bullets were used to drive out the protesters at the time.

Criticized for participating in the controversial event, Esper said that he was not aware of the plan to take the group picture at the church, nor did he know protesters were being dispersed when the group made their way. "I did know that we were going to church, I was not aware that a photo op was happening."

James Miller, a member of the Pentagon's Defense Science Board, resigned in response to the incident. "You may not have been able to stop President Trump from directing this appalling use of force, but you could have chosen to oppose it. Instead, you visibly supported it," he told Esper in his resignation letter dated Tuesday.

As violent confrontations between protesters and police seemed to subside in Washington D.C., active-duty troops deployed to the nation's capital have started returning to their home bases on June 3, U.S. media reported.

Esper said at the news briefing that his goal was "to keep the department out of politics and stay apolitical."