WASHINGTON (AP) — The coronavirus is taking a growing toll on the U.S. military, and commanders and senior officials are bracing for worse. From nuclear missile fields at home to war zones abroad, from flight lines to ships at sea, the Pentagon is striving to shield vital missions even as it faces urgent calls for help on the civilian front.

Training exercises big and small, including one of the largest in Europe since the end of the Cold War, have been curtailed. Army recruiting stations have closed. Troops around the globe are hunkering down to confront an enemy unlike anything the world's most lethal armed forces have encountered before.

"It's unprecedented in my lifetime," Defense Secretary Mark Esper said, adding that he believes that "in a period of months" the worst will be over and the force will once again be "fully mission capable."

Until then, the ramifications of COVID-19 for the military are likely to expand. There are worries, for example, about the defense industry being weakened and key weapons development slowed.

Gen. Mark Milley, chairman of the Joint Chiefs of Staff, says that in a worse-case scenario, the military will lose some of its preparedness for combat and other missions. But the impact is likely to be "moderate to minor to low in terms of its risk" to national defense, he told reporters on Monday.

"I'm just not in a place right now where I can give you an accurate description of what that's going to look like," he said.

The Pentagon budget also is taking a hit, although a coronavirus relief bill working its way through Congress would provide billions for the growing list of new expenses, including loans to industry that would enable accelerated production of medical equipment like ventilators and respiratory masks for civilian needs.

Many of America's closest allies are waging their own COVID-19 battles, including Britain, Germany, Italy, France and other NATO partners, as well as South Korea, which hosts about 28,500 U.S. troops. At least temporarily, the pandemic has taken the edge off conflict in some of the globe's major flashpoints, such as Iran, as governments focus on fighting the disease inside their own borders.

Gen. Joseph Lengyel, chief of the National Guard Bureau, which is accustomed to responding to single-event domestic natural disasters like a flood or hurricane, said the coronavirus outbreak is akin to having hurricanes of varying ferocity hit every U.S. state and territory, as well as the District of Columbia.

"This is a historic event that will require a historic response," Lengyel said.

The Guard has been mobilized by every state governor and in three territories. In Washington, California and New York the federal government is footing the bill. As of Wednesday, more than 10,000 Guard members were on duty. The active-duty military also is helping, with three field hospitals deploying to New York and Washington, and Navy hospital ships expected in Los Angeles and New York.

The Guard is chipping in with a range of expertise, including what it calls weapons of mass destruction teams that are helping set up drive-thru testing stations.

No U.S. military member is known to have died from COVID-19, although infections are spreading.

Brig. Gen. Paul Friedrichs, the top doctor on the staff of the Joint Chiefs of Staff, said Wednesday it appears highly likely that the spread inside the military will continue. Over the next three weeks, which he said is as far ahead as modeling data can reliably support a forecast, "we think we're going to see this — no surprise — continue to grow." His comment aligns with those of many others in the medical community and across the military, standing in contrast to President Donald Trump's statements about starting to return the country to normalcy by Easter.

As of Thursday, 280 of the 1.4 million-strong active duty force had been confirmed with the disease, up from 51 a week earlier.

"Our curve is not flattening," Friedrichs said.

The virus has penetrated the Pentagon, but with modest impact thus far as Esper and Milley remain at work in the military headquarters. As a precaution, Esper and the deputy defense secretary, David Norquist, are being kept physically separated. On Monday, the building's health risk alert level was raised a notch, and on Wednesday Esper ordered the higher alert level for all Defense Department sites worldwide.

In a virtual town hall meeting with Defense Department employees, Esper on Tuesday said his top priority is protecting troops and their families, but he noted that the military cannot function free of risk.

"You can't do social distancing in a submarine or even a tank," he said.

Navy ships have stopped port visits abroad, except when resupply or maintenance requires it. On Thursday the Navy said eight sailors aboard the USS Theodore Roosevelt tested positive after the aircraft carrier made a port visit in Vietnam. This was the first reported time the virus has struck a warship at sea. The carrier was being diverted to Guam on Thursday, and all 5,000 aboard are to be tested. In general, the virus has been slower to disable a generally younger and healthier military population.

Not every ounce of normalcy has drained from the military establishment, but many of the problems and projects that dominated the defense scene just weeks ago have lost some of their immediacy — the ongoing battles against the Islamic State group, the drawing down of troops in Afghanistan, the stand-up of a Space Force, the grand plan for shifting the military's focus toward China.

This sudden shift has echoes of September 2001. One day before the terrorist attacks, then-Defense Secretary Donald Rumsfeld was making a public pitch for combating bureaucratic waste in the Pentagon, a campaign that fell to the wayside as the nation geared up for war against al-Qaida and the Taliban.

Military commanders have been taking steps to protect and isolate key troops such as those on counterterrorism teams, security forces and air crews. Officials acknowledge growing concern about U.S. troops in Iraq and Afghanistan, where thousands of locals who had been in neighboring Iran have streamed back across borders.

So far, no U.S. service member in Iraq, Syria or Afghanistan is known to have tested positive for the virus, but a couple dozen have shown symptoms and are in isolation. Navy Capt. Bill Urban, spokesman for U.S. Central Command, said two U.S. sailors tested positive in Bahrain earlier this week. Across the Mideast, about 1,500 U.S. troops are in precautionary quarantine.

In Iraq, commanders have moved U.S.-led coalition troops off smaller bases and sent some out of the country after the Iraqi government suspended all military training.

Though little discussed in public, the U.S. has made significant adjustments to ensure that the nation's strategic nuclear forces — the bombers, submarines and land-based missiles that form the U.S. nuclear "triad" — remain at the ready. For example, airmen who operate and secure Minuteman 3 nuclear missiles on the Great Plains are doing longer tours in the missile fields and self-isolating afterward.

The Navy has paused some phases of initial training for its elite SEAL force. Navy Capt. Tamara Lawrence, a spokeswoman for Naval Special Warfare Command, said an eight-week pause is affecting three training sections. She said early training that pushes new candidates to the limit was paused because the extremely strenuous requirements can risk affecting their immune systems.

Lawrence said there is no current expectation that the pause will affect the Navy's ability to fill its need for SEALs and the personnel who drive their boats.

The Army's specialized Green Berets are continuing field training at Fort Bragg, North Carolina, although some classroom training has been shifted online.

For most people, the new coronavirus causes mild or moderate symptoms, such as fever and cough that clear up in two to three weeks. For some, especially older adults and people with existing health problems, it can cause more severe illness, including pneumonia and death.