Army Maj. Gen. Darryl Williams, commanding general of U.S. Army Africa, speaks to U.S. Navy Seabees at the construction site of  a 25-bed U.S.-built hospital for Liberian health workers as part of Operation United Assistance on Oct. 9. (John Moore/Getty Images)

The U.S. military continues to grow the force it is deploying to western Africa to assist with the Ebola virus crisis, but it will take until late November or early December to complete all 17 treatment units it has planned, a two-star general said.

Army Maj. Gen. Darryl A. Williams, commander of U.S. Army Africa, told reporters in a phone conference from Liberia on Tuesday that the “lion’s share” of the treatment units will be complete by late November, with a few lagging into December. That exceeds an estimate provided by his commanding officer, Gen. David Rodriguez, who said Oct. 7 that the effort would likely take until mid-November.

Williams has been on the ground about 30 days in Liberia, working from a headquarters in the capital  of Monrovia. The Pentagon has said that up to 4,000 U.S. personnel could deploy, primarily from the U.S. Army. A force that includes soldiers, U.S. Marines from a crisis-response unit based in Spain and Navy Seabees already has deployed, and will grow in coming days.

The new Ebola treatment units are a centerpiece of the U.S. assistance effort. U.S. officials have said they will each include 100 beds, and be based not only in Monrovia, but in more remote locations north and south of the city. The first is close to completion, Williams said. The effort has been hampered by heavy rains, among other obstacles.

The Pentagon’s effort is about to enter its next phase. Maj. Gen. Gary J. Volesky, commanding general of the 101st Airborne Division, of Fort Campbell, Ky., is expected to arrive in Monrovia in the next few days and replace Williams as the top U.S. commander, allowing Williams to return to his role overseeing Army units across Africa. The Army expects to eventually deploy about 3,200 personnel, officials said.

The U.S. military also has deployed three mobile laboratories to assess whether individuals with symptoms actually have the Ebola. U.S. troops with training in infectious diseases test samples for presence of the virus, cutting the time it takes to determine whether a patient is infected from a few days to a few hours, Williams said.

The United States has faced criticism for how long it has taken for it to send troops, but Williams defended the plan  Wednesday. It made sense to perform on-the-ground assessments before growing the force, he said.

“It absolutely was the right thing to do, and we have the right capacity to do that,” he said of the deployment plan.

Most U.S. troops already involved in the effort are focused on building facilities that are needed and on preparing an intermediate staging base in Senegal that will funnel medical equipment and other supplies into infected areas. Only a small number of highly trained service members who work with infectious diseases will wear protective equipment to ward off germs, Williams said.

The Pentagon has adopted a series of protocols for its troops deploying, however. Their temperature is taken multiple times per day to look for symptoms, and Williams left open the possibility that commanders could impose a quarantine on them when they return to the United States until it is certain that they have not been infected.

“We are monitoring ourselves every single day, and commanders always have the ability to be more conservative,” the general said.

If a service member gets the virus, they will be brought back to the United States for treatment on a specially designed plane that isolates them, Rodriguez, the commander of U.S. Africa Command, said last week.