Four of Aberdeen's six full-time fire appliances were declared "off the run" for a period last week, BBC Scotland has learned.
It is understood there were not enough officers to crew the fire engines at the start of the day shift last Thursday.
The Fire Brigades Union said it was concerned over what happened.
After an hour, enough firefighters were found to crew two more appliances.
However, between 08:00 and 18:00 the two fire engines based at Altens in the south of Aberdeen were not operational.
Gary Lennie, the north area secretary of the FBU, said: "I'm concerned this has happened in Aberdeen City.
"We have the three whole-time fire stations in Aberdeen and we want the crewing for the appliances to be covered 24/7.
"If we had had to attend one of the more than 60 high-rise buildings in Aberdeen, we would have been really stretched."
Bruce Farquharson, the Scottish Fire and Rescue Service's local senior officer for Aberdeen City, said: "We can confirm that two appliances at Altens Community Fire Station were off the run from 8am to 6pm on Thursday, March 29.
"For one hour on the same day, an additional two pumps were off the run in the Aberdeen City area.
"At no point was firefighter or community safety compromised. The community was fully protected through our robust strategic planning which ensured cover was maintained at all times."
He added: "The Scottish Fire and Rescue Service continues to attend at every emergency."