Beijing's new US$63 billion Daxing airport began its first scheduled international flights on Sunday as it ramped up operations to help relieve pressure on the city's existing Capital airport.

Shaped like a phoenix - though to some observers it is more reminiscent of a starfish - the airport was designed by famed Iraqi-born architect Zaha Hadid, and formally opened in late September ahead of the Oct. 1 celebrations of the 70th anniversary of the People's Republic of China.

It boasts four runways and is expected to handle up to 72 million passengers a year by 2025, eventually reaching 100 million.

China Southern Airlines and China Eastern Airlines will be the main domestic carriers at Daxing, though Air China will provide a small number of flights too.

An Air China flight to Bangkok was the first international flight to leave on Sunday, while British Airways will operate the first transcontinental flight, to London.

About 50 foreign airlines, including Finnair, plan to move all or part of their China operations to the airport in the coming quarters.

The relocation of all the airlines which will use Daxing is to due to be completed by the winter of 2021. Air China and its Star Alliance partners will remain mostly at Capital airport.

The airport, roughly the size of 100 football fields and expected to become one of the world's busiest, has come in for some criticism due to its distance from central Beijing.

By public transport it takes over an hour to reach it from Beijing's central business district, more than double the time needed to reach Capital airport, which strains at the seams and is often hit by delays.

Officials say Daxing airport is not only designed to serve Beijing, but also the surrounding province of Hebei and next-door city of Tianjin, to boost regional development.