Fossil Free Football say: "Particular concern is for the fixtures in the no-shade stadium in Charlotte where a heat index of 38C (Real Madrid v Pachuca) and 41C (Benfica v Bayern) is forecast for the next two matches.
"This underlines the dangers facing players and fans at this tournament and in 2026. Fifa have done very little to allay safety concerns."
The game between New Zealand part-timers Auckland City and Boca Juniors is also due to take place in 41C in Nashville at a ground with limited cover.
Matches in Philadelphia, New York, Cincinnati and Washington DC will also be played in the anticipated heatwave.
After Atletico's 4-0 defeat by Paris St-Germain in 32C in Pasadena, Marcos Llorente said it was "terribly hot" and added "my toes were sore, my nails were hurting... it's incredible".
Fans spoke of having to leave that game because of the heat - and complained about long queues and restrictions of water when arriving at the stadium.
A Fifa statement read: "Fifa's top priority is the health of everyone involved in football, and Fifa's medical experts have been in regular contact with the clubs participating to address heat management and acclimatisation."
The governing body added there will continue to be cooling breaks in the 30th and 75th minute where needed - and that fans are allowed to bring empty clear bottles of up to one litre into stadiums.
Chelsea, who are one of two English teams competing along with Manchester City, beat Los Angeles FC in their opener but then lost 3-1 to Flamengo on Friday.
"It's not easy because of the temperature," said boss Enzo Maresca. "We're going to try to rotate players."