U.S. Navy Aircrewman 2nd Class Samuel Sellato observes during an aerial assessment of military installations affected by Hurricane Florence over North Carolina on Sept. 16, 2018. (Michael R. Sanchez/USS Kearsarge/Reuters/U.S. Navy)

Catastrophic flooding spawned by former hurricane Florence is expected to worsen in coming days, prompting the U.S. military to keep thousands of troops prepared to help and begin flying aircraft from Navy ships nearby to survey the worst areas, a U.S. general said.

Air Force Gen. Terrence J. O’Shaughnessy, the chief of U.S. Northern Command, said Monday that the Marine Corps’s MV-22 Osprey aircraft began flying off the ships in the past day. They will be on call along with heavy-lift CH-53 helicopters potentially through Thursday, when swollen rivers are expected to reach their peak heights.

“Right now, what we’re concerned about is the growing concern for isolated areas,” O’Shaughnessy said. “As the rivers rise and potentially communities are cut off, we think having this capability available will be important to make sure we properly supply and bring in support as required.”

About 5,600 active-duty U.S. service members are involved in the effort, along with 6,700 National Guardsmen, O’Shaughnessy and Air Force Gen. Joseph L. Lengyel, the chief of the National Guard Bureau, said in a joint phone interview. Army Gen. Jeffrey S. Buchanan, the commander of U.S. Army North, has been leading the effort in the Carolinas and will continue visiting afflicted areas, O’Shaughnessy said.

The Pentagon’s effort involved about 13,000 U.S. troops over the weekend, but the number could rise if river flooding is substantial, Lengyel said. Some of the guardsmen who were activated to respond in South Carolina have since been released from their duties.

The Coast Guard, which does not report to the Pentagon, has at least 3,000 additional service members involved, officials said Sunday. Coast Guardsmen have carried out rescues with helicopters and 35 small shallow-water boat teams deployed in North Carolina. Nine helicopter crews based in Savannah, Ga., searched the Georgia and South Carolina coastline Sunday looking for anyone who needed help and shifted afterward to fly operations this week from Myrtle Beach, S.C., over North Carolina.

The Pentagon’s effort is a sprawling affair that spans bases from Alabama to Delaware. It’s aided in part by the fact that there are numerous military installations in North Carolina, including the Army’s Fort Bragg and the Marine Corps’s Camp Lejeune. Marine officials said in a Facebook post Monday that nonessential personnel are not required to report to their jobs until Sept. 24, while all employees at Fort Bragg were directed to report Tuesday.

North Command officials said Monday that they dispatched two surveillance planes over the Carolinas to assess bridges, roads and other infrastructure by air to help prioritize flood relief efforts. They also have more than 1,000 high-water vehicles available to traverse flooded areas, and dozens of helicopters available to move people, equipment and supplies.

Before the storm, Defense Secretary Jim Mattis directed military commanders to be ready to provide anything that was needed to the Federal Emergency Management Agency and local government officials.

“Clearly we saw the potential for some extreme conditions and wanted to make sure that we were fully prepared and postured,” O’Shaughnessy said.