Cathay Pacific said Friday it has ordered 21 long-haul Boeing 777-9X planes at a list price of $7.48 billion dollars, as the Hong Kong carrier modernises its fleet off the back of disappointing earnings this year.

The yet-to-be launched 777X series is Boeing's newest member of the 777 family, with advanced technology including composite wings and new engines that Boeing says consume 20 percent less fuel than today's model.

Cathay did not reveal the amount it is paying to the US manufacturer, but airlines usually negotiate a discount from a plane's catalogue price.

"The 777-9X promises us improved payload range capability and reduced operating costs, in addition to a significant reduction in environmental emissions," the airline's chief executive John Slosar said in a statement.

"Cathay Pacific is committed to modernising its fleet to provide a superior experience to passengers," Slosar said, adding that the airline was "delighted to be an early customer" for the next generation of the 777 series.

The airline said the planes, which will seat up to 400 passengers and will be delivered between 2021 and 2024, were ideal for long-haul destinations in North America and Europe.

"The huge investment we are making in new aircraft underscores... our commitment to maintaining Hong Kong's position as one of the world's great aviation hubs," Slosar said.

The airline said in August it swung to a lower-than-expected net profit of $3.1 million in the first six months, after losing money in the same period of 2012. Gains in passenger numbers were offset by persistently high fuel prices and falling cargo revenue.

The high price of jet fuel had affected its business adversely, Cathay said in a statement then, adding that it was withdrawing older planes and operating more long-haul services using fuel-efficient Boeing 777-300ER aircraft.

The airline faced a difficult year in 2012 when profit plunged more than 83 percent over the full 12 months, buffeted by high fuel prices and the global financial crisis.

Starting from 2014, the airline is expecting the delivery of 12 Boeing 777-300ERs, and will see 50 of this aircraft type in its fleet by 2020.

This year, Cathay took delivery of six new aircraft in the first six months, including two Airbus A330-300s, three Boeing 777-300ERs, and one Boeing 747-8F freighter.

It has increased daily services to popular destinations such as London, Los Angeles, New York and Toronto.

The airline Emirates placed an order in November for 150 Boeing 777X aircraft -- 35 777-8Xs and 115 of the larger 777-9X variant, in a contract the Dubai-based carrier said was valued at $76 billion.

Boeing said the Emirates orders were commitments worth $55.6 billion.

Abu Dhabi's Etihad Airways last month placed an order for 25 777X passenger jets, including 17 777-9X models and eight 777-8Xs.