A World War II-era bomber plane has crashed in a fireball after taking off at a busy Connecticut airport, killing at least five of the 13 people on board.

The four-engine, propeller-driven B-17 plane struggled to get into the air and slammed into a maintenance shed at Bradley International Airport at Hartford as the pilots circled back for a landing, officials and witnesses said.

It had 10 passengers and three crew members, authorities said.

A state official who spoke on condition of anonymity gave the death toll as five.

An anonymous offical puts the death toll at 5.

Connecticut public safety commissioner James Rovella said some of those on board suffered severe burns and “the victims are very difficult to identify”.

Governor Ned Lamont said his heart went out to the families who were waiting for news of their loved ones.

“And we are going to give them the best information we can as soon as we can in an honest way,” he said.

Antonio Arreguin was parked at a construction site near the airport when he heard an explosion and felt the heat from the flames, about 230 metres away.

“I see this big ball of orange fire, and I knew something happened,” he said.

– AAP