The Pentagon is defending Defense Secretary Mark Esper's use of the term "battle space," following criticism from retired military officers and congressional leadership about the term and the possible domestic deployment of the active duty military.

In a call with President Donald Trump and governors on Monday -- a recording of which was obtained by ABC News, Esper urged states "to dominate the battle space" so that civil unrest "dissipates and we can get back to the right normal."

Over the last week, thousands of Americans have peacefully protested systemic racial inequality sparked by the death of George Floyd at the hands of a Minneapolis police officer. But looting and riots in many American cities have caused states to activate the National Guard to support law enforcement -- with the president threatening to deploy the active duty military to quell the violence.

Two retired four-star generals took the rare step of publicly condemning Esper's comments on Monday, arguing the language was inappropriate to describe the current situation.

"The 'battle space' of America???" tweeted ret. Army Gen. Tony Thomas, who led U.S. Special Operations Command. " Not what America needs to hear...ever, unless we are invaded by an adversary or experience a constitutional a Civil War..."

Former Chairman of the Joint Chiefs of Staff Gen. Martin Dempsey expressed a similar sentiment, tweeting, "America is not a battleground. Our fellow citizens are not the enemy."

Asked about Esper's use of the term "battle space," a senior defense official, who declined to be identified, told Pentagon reporters on Tuesday that the "DOD often communicates in a parlance unique to the profession of arms."

"He was using the terms that we have," the official said. "Nothing should be read into the use of that term to denote anything other than it's a common term to denote the area we are operating in."

Esper served 10 years in the U.S. Army on active duty and another 11 years in the National Guard and Army Reserve.

A former senior administration official told ABC News that congressional leaders on both sides of the aisle have privately expressed concerns about Esper's words because they could be seen as laying the groundwork for the invocation of the Insurrection Act of 1807, which allows the president to deploy active duty troops within the United States under limited circumstances. The president would have to invoke the act because the active duty military -- unlike the National Guard -- is barred from carrying out law enforcement activities under Posse Comitatus Act.

On Tuesday, Rep. Adam Smith, D-Wash., chairman of the House Armed Services Committee, called on Esper and Chairman of the Joint Chiefs of Staff Gen. Mark Milley to testify before his committee next week to explain the role they envision the U.S. military playing in response to protests.

He also told reporters that language like "battle space" is "deeply concerning in terms of how the U.S. military would be used for domestic law enforcement," though he had not personally spoken to Esper about it.

As of Tuesday, governors in 28 states and the District of Columbia had activated more than 20,400 National Guard members to assist state and local law enforcement operations, according to the National Guard.

But overnight, elements of the 82nd Airborne Division, along with active duty military police units from Fort Bragg, Fort Drum and Fort Riley, staged at Joint Base Andrews outside of the nation's capital in case they were requested, according to two U.S. officials.

"The decision to use active military forces in crowd control in the United States should only be made as a last resort," said ABC News analyst Mick Mulroy, a retired Marine and former deputy assistant secretary of defense for the Middle East.

"The National Guard is fully capable and trained for these operations. They are from these communities and likely have been activated many times to support people after fires, hurricanes, or pandemics. Some of the people they helped are the ones protesting. Active Army and Marine Corps units are trained to fight our nation's enemies, not their fellow Americans."

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