Qatar on Thursday opened a huge expansion of its main international airport as the national carrier's chief hit back at "rumours" that it could not cope with World Cup passengers.

After the multi-billion dollar overhaul, Hamad International Airport will take its capacity from 40 million passengers a year to 58 million, Qatar Airways chief executive Akbar Al Baker told a news conference.

The expansion adds 39 airport gates to the existing 140 at the airport, which is a major hub for transit passengers.

The project was completed just 10 days from the start of the World Cup, which is expected to draw more than one million visitors to the Gulf state, the smallest ever host of the mega event.

Qatar has reopened Doha International Airport to ease pressure.

Qatar Airways has also eliminated 18 destinations from its schedule to increase flights from key World Cup markets and to allow other airlines to run football charters.

Baker dismissed suggestions that Qatar's aviation authorities could not cope.

"There have always been rumours," he said before insisting that "systems and innovations" introduced for the World Cup had been tested.

"We have already made sure that all the capacity involved with extra flights and charter flights meet the maximum capacity per hour of each of the two airports," he said.

"So we are very well positioned in order to cater for this very large influx of passengers."

At the centrepiece of the new terminal expansion is an indoor tropical garden featuring more than 300 different trees and 25,000 plants from around the world.

A new phase of the expansion will start in January, aiming to take the capacity at Hamad Airport -- which only opened in 2014 -- to more than 70 million passengers a year.

"What we have done in this very short period of time already has raised the bar that I doubt that any airport ever will be able to match," the airline chief declared.