The Pentagon has stopped providing details about the spread of coronavirus through military ranks over fears that the information could be weaponised by US adversaries.

Defence Secretary Mark Esper announced the decision one day after the Pentagon revealed its first confirmed Covid-19 case involving a Marine stationed at the department's headquarters in Arlington, Virginia.

Secretary Esper told Reuters on Thursday that the department will provide "aggregated numbers" but that officials will not "disaggregate numbers because it could reveal information about where we may be affected at a higher rate than maybe some other places."

The virus may cause adversaries to "act out in different ways" as it progresses and cycles through different countries, he said at a news conference on Wednesday. "We need to be very cautious and be on the lookout for that. ... We have to be looking out beyond that and how it may be affecting our mission."

Limited data about the number and rates of infections among service members would reverse course from the routine updates from military officials about the locations of each case, which included warships abroad and inside the Pentagon itself.

A defence spokesperson said that the department is "committed to transparency" but should the coronavirus "affect the readiness of our strategic deterrent or strategic response forces, we would understandably protect that information."

Secretary Esper also elevated the Health Protection Condition to its second-highest designation (Charlie), indicating "sustained community transmission" of an infectious disease.

Air Force Brigadier General and joint staff surgeon Paul Freidrichs said that the designation reflects that the "curve is not flattening" and more stringent restrictions are necessary.

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Secretary Esper said: "I'm not going to get into a habit where we start providing numbers across all the commands and we come to a point six [or] seven weeks from now where we have some concerns in some locations and reveal information that could put people at risk."

There are more than 600 cases of the virus among service members, civilians and contractors working with the military, including 280 cases among US troops.

This week, officials with the US Army, the largest branch of the military, initially paused training and nonessential military exercises and activities to prevent close contact among troops but changed their minds despite cases of the virus in the military doubling within days.

The department also ordered a freeze on all US military travel for up to 60 days to prevent the spread of the virus.

Officials have committed to reducing the number of troops in Afghanistan from 13,000 to 8,600 within 135 following a landmark deal with the Taliban last month. Full withdrawal could take place within a year, provided that the Taliban commit to its end of the deal. Secretary

Esper said the withdrawal will not be impacted by the latest order.

Roughly 9,000 National Guard service members are deployed throughout the US to assist states in their response to the pandemic.