Eight armed and masked men made a hole in a security fence at Brussels' international airport, drove onto the tarmac and snatched millions of dollars' worth of diamonds from the hold of a Swiss-bound plane without firing a shot, authorities said on Tuesday.

The gang used two vehicles in their daring raid on Monday, grabbed the cache of stones and sped off into the darkness, said Anja Bijnens, spokeswoman for the Brussels prosecutor's office.

Baggage carts make their way past a Helvetic Airways aircraft from which millions' of dollars worth of diamonds were stolen on the tarmac of Brussels international airport on Tuesday. Yves Logghe / Associated Press

Police found a burned-out vehicle close to the airport later on Monday night and said they were still looking for clues.

The heist was estimated at about $50 million in diamonds, said Caroline De Wolf of the Antwerp World Diamond Centre.

"What we are talking about is obviously a gigantic sum," De Wolf told the VRT network.

An airport spokesman said the robbers made a hole in the perimeter fence and drove up to the Swiss passenger plane, which was ready to leave. The robbers got out of the car, flashed their weapons and took the loot from the hold, said airport spokesman Jan van der Cruysse. Without firing a shot, they drove off through the same hole in the fence, completing the spectacular theft within minutes, he said.

Van der Cruysse could not explain how the area could be so vulnerable to theft. "We abide by the most stringent rules," he said.

The Swiss flight, bound for Zurich and operated by Helvetic Airways, was canceled. Swiss, an affiliate of Germany's Deutsche Lufthansa AG, declined to comment on the heist, citing the ongoing judicial investigation.

The insurance for air transportation - handled sometimes by airlines themselves or external insurance companies - is usually relatively cheap because it's considered to be the safest way to move small high-value items, logistics experts say.