America's biggest airlines have called on Washington for more than $50bn in aid as they suffer the effects of the coronavirus pandemic.

US President Donald Trump on Monday pledged to back the industry "100%".

But figures suggest major carriers spent almost all their spare cash in the last decade buying back their own shares.

It comes as the airline industry is facing huge losses and widespread layoffs as passengers stay at home.

Data revealed by Bloomberg show major airlines including United and American Airlines used up an average 96% of their spare cash buying back their own shares.

Companies buy back their own shares for a number of reasons. Some have built up big cash piles that they don't want to sit on so spend the money buying back previously issued shares. This helps them reduce their costs as they have fewer shareholders to pay dividends to.

Buying back stocks can also push up the company's share price, which many investors use to measure a company's performance.

American Airlines led the pack, buying back more than $12.5 billion of its own shares from 2010 to 2019, according to Bloomberg figures. United Airlines used 80% of its spare cash buying back its shares.

The average S&P 500 Index company spent about 50% of its spare cash buying back its own shares during this period.

With limited cash reserves and a significant drop in revenues expected, US airlines are looking for government assistance of more than $50bn. Plane maker Boeing is also one of the firms looking for short-term assistance.

US Treasury Secretary Steve Mnuchin said last week that airlines are "on top of the list" for government help.