LONDON (BLOOMBERG) - Airlines are piling on more debt as surging coronavirus cases force travellers to cancel plans and stay home.

The industry's outstanding debt has jumped 23 per cent since last year to US$340 billion (S$456 billion), according to data compiled by Bloomberg.

So far this year, global air carriers have sold US$63 billion in bonds and loans.

It is more evidence that the industry faces a bumpy road ahead, with many border restrictions still in place and the high-season of summer vacations in the United States and Europe coming to an end.

EasyJet and Japan Airlines announced new fund-raising plans this month to help them weather the prolonged pandemic.

Ms Susannah Streeter, a senior investment and markets analyst at Hargreaves Lansdown, said: "The spread of the Delta variant may lead to other countries imposing tougher quarantine rules on visitors."

Many carriers are returning to the bond market after last year's dash-for-cash when the pandemic first struck. The big sales show that investors are still ready and willing to give ample funding to the industry.

Some recent examples of companies in the market are EasyJet, which raised US$400 million of new debt along with £1.2 billion (S$2.2 billion) in stock. The funding will provide a buffer to get through the slow winter season and position the carrier for a tentative rebound in leisure travel.

Japan Airlines secured almost 300 billion yen (S$3.7 billion) in fresh funding via subordinated bonds and loans. The funds will be used to upgrade its fleet by procuring Airbus A350-1000 aircraft as its flagship for international lines, it said.

Australia's Qantas Airways has announced plans for a bond sale, while the European Union's latest pandemic recovery bond sale is expected to spur a hectic week of sales from Europe's safest borrowers.