The camp was originally scheduled to take place in early July in Chiba, a prefecture neighbouring Tokyo.
The US track and field team led the athletics medal table at the last Olympic Games in Rio in 2016, with 13 golds, 10 silvers and nine bronzes. Kenya were second with six, six and one.
The Tokyo 2020 Olympic Games were postponed last year are scheduled to take place from July 23. The absence of the US team would be a huge blow for the organisers who would fear a domino effect.
Rafael Nadal, meanwhile, has joined the growing number of tennis players expressing doubts about competing in Tokyo.
“I don’t know yet. Honestly, I can’t give you a clear answer because I don’t know,” Nadal told the Associated Press.