Senate Democrats are pressing U.S. Secretary of Defense Mark Esper over concerns about the Pentagon's response to the COVID-19 pandemic.

In a Wednesday letter to Esper, nine senators called reports of a rise in cases among service members in July "concerning."

The number of coronavirus infections reportedly rose by more than 21,000 that month -- an increase of more than 100%.

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According to the department's website, as of Wednesday, the Pentagon had reported a total of 53,033 coronavirus cases with 80 deaths. Some 36,600 of those cases involved members of the U.S. military, and six service members were included in the death toll.

The Hill reported Thursday that of the troops who have died, one was an active-duty sailor while the others were reservists or members of the National Guard.

“We are pleased to see that the department is taking some precautionary measures to address the spread of the virus, but are concerned that the department is still not properly prioritizing the health and well-being of our service members,” the senators wrote.

The letter was signed by Massachusetts Sen. Elizabeth Warren, Hawaii Sen. Mazie Hirono, Washington Sen. Patty Murray, Connecticut Sen. Richard Blumenthal, Oregon Sen. Jeff Merkley, Ohio Sen. Sherrod Brown, Oregon Sen. Ron Wyden, Massachusetts Sen. Ed Markey and Minnesota Sen. Amy Klobuchar.

Defense Secretary Mark Esper discusses the department's response to the COVID-19 pandemic in a March news conference. (Army Staff Sgt. Nicole Mejia/Department of Defense via AP)

The same group of senators -- plus California Sen. Kamala Harris -- sent a similar letter in April expressing “grave concern," sparking a fierce response from the Pentagon.

Esper himself issued a sharp rebuke during a news conference and sent a letter to Senate Armed Services Committee Chairman Jim Inhofe, an Oklahoma Republican.

A letter released Thursday by Warren's office revealed the Pentagon had also replied directly to the senators in May.

“It is unfortunate that your letter used inaccurate media reports that have been discredited to unfairly portray the department while we are in the middle of the COVID-19 fight,” then-Assistant Secretary of Defense for Legislative Affairs Robert Hood admonished.

“To be clear, what we have done is work with some of the leading health care experts in the military and throughout the government to find the right balance of protecting our people and protecting America,” he penned.

In the Democratic senators' Thursday letter, they wrote they were “surprised” Esper would “deflect [their] concerns about the COVID-19 pandemic and its impact on readiness and morale.”

They said that their concerns are with the department's leadership, noting that cases are "rising at an alarming rate."

The politicians also pointed out that the May correspondence from Hood did not answer their specific questions regarding the department's plan to address the pandemic while continuing operations.

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Congress stands ready to support the department, but we cannot do so if basic questions are not answered regarding DoD’s response,” they wrote.

The Pentagon hasn't yet responded to a request for comment from Fox News.