Brussels Airlines, Belgium's top carrier, said Tuesday it was facing its "biggest crisis" ever as last week's airport bomb attacks cost it five million euros a day in lost business.

Two Islamic State suicide bombers blew themselves up at Zaventem airport on March 22, with authorities warning it will take months to restore normal services although some flights may be able to start Wednesday if tests of temporary facilities go well.

"We are going through the biggest crisis of our history," Brussels Airlines spokeswoman Wenke Lemmes told AFP as the company waited anxiously for news from the airport.

Lemmes said that only 20 of the airline's 50 aircraft were currently operating and it was carrying just 5,000-6,000 passengers compared with more than 22,000 on a normal day.

"We are losing about five million euros ($5.5 million) a day but we remain optimistic. Once we get back to full service, the passengers should come back," she said.

Brussels airport is a major European hub and has flights to destinations worldwide.

Chief executive Arnaud Feist warned Tuesday that even if all the tests went well, "at best" about 20 percent of services might be restored on Wednesday.

For a full return to normal, "we will have to wait for months," Feist told L'Echo daily.

Brussels Airlines was formed in 2002 after the collapse of Belgian national carrier Sabena but it has struggled to establish itself and after years of losses only returned to profit in 2015.

Germany's Lufthansa holds 45 percent of the airline and, after the bomb attacks, Brussels Airlines transferred some aircraft to German airports.

Other airlines switched to regional airports in Belgium or transferred to neighbouring countries such as the Netherlands and Germany.