The US Department of Transportation is investigating Delta Air Lines over reports that passengers were forced to wait on an idle plane in extreme heat.

The plane in question sat on the tarmac at the Las Vegas airport for at least three hours on Monday as temperatures rose as high as 111F (44C).

Multiple passengers were seen by medical first responders and two people went to a local hospital.

Delta says it has offered compensation and apologised to affected passengers.

“I want to know how it was possible for passengers to be left in triple-digit heat onboard an aircraft for that long,” Transportation Secretary Pete Buttigieg told Reuters on Thursday.

He called the situation “infuriating” and “shocking”.

“Even under normal temperatures a tarmac delay is not supposed to go that long and we have rules about that, which we are actively enforcing and this is being investigated right now,” he continued, adding that airlines are required to maintain comfortable cabin temperatures.

The flight bound for Atlanta was ultimately cancelled after customers sat in the un-air conditioned plane for three hours.

Fox News field producer Krista Garvin, who was on board the plane, told Fox Business earlier this week that passengers were forced to wait with no air conditioning as a number of passengers fell ill, as did some flight crew members.

“Delta teams are looking into the circumstances that led to uncomfortable temperatures inside the cabin and we appreciate the efforts of our people and first responders at Harry Reid International,” the airline said this week.