There have been bitter recriminations over the almost week-long closure of large parts of European airspace because of volcanic ash from Iceland.
Airlines are seeking compensation from governments over the disruption, said to be the worst since World War II.
But scientists have said regulators had few options beyond a ban on flights.
Although officials said air traffic in Europe would be back to "almost 100%" on Thursday, certain airports in Sweden and Norway reportedly closed again.
Airports in southern Sweden Gothenburg's Landvetter and Malmo's Sturup closed late on Wednesday when part of the volcanic ash cloud took an unexpected turn north, Sweden's TT news agency reported.
It said the airports would remain closed on Thursday morning but may open later in the day.
Activity at Charles de Gaulle Airport in Paris was back to normal by Thursday morning, officials said, with most departures leaving on time except for a number of delayed flights to the UK.
Tens of thousands of people remain stranded around the world as airlines restart services after the unprecedented travel chaos.
'Poor decision-making'
Six days after the eruption of the Eyjafjallajokull volcano in Iceland triggered the first airspace closures, the International Air Transport Association (IATA) said airlines had lost $1.7bn (£1.1bn).
For an industry that lost $9.4bn last year and was forecast to lose a further $2.8bn in 2010, this crisis is devastating," IATA chief Giovanni Bisignani said. "Airspace was being closed based on theoretical models, not on facts."
I think if they'd sent up planes immediately to see whether the ash was actually too dangerous... we would have been back flying a lot sooner
Sir Richard Branson
Chairman, Virgin Group
Mr Bisignani said the situation had been exacerbated by "poor decision-making" from governments and called on them to compensate airlines, something which the European Commission has said it is considering.
"I am the first one to say that this industry does not want or need bailouts. But this crisis is not the result of running our business badly," he added.
"Governments should help carriers recover the cost of this disruption."
Compensation row
Some airlines are also demanding changes to EU passenger compensation rules, which require them to provide accommodation for those prevented from flying.
Michael O'Leary, the chief executive of the low-cost carrier Ryanair, said it was "absurd" that his firm had to spend thousands of euros on someone whose ticket might have cost only a few euros.
Mr O'Leary said Ryanair would only reimburse travellers the original price of their airfare and no more, potentially setting up a clash with Ireland's Commission for Aviation Regulation and the EU.
"We will look forward to seeing them in court because frankly I think this is a great opportunity for airlines to expose this nonsense," he added.
Virgin Group chairman Sir Richard Branson meanwhile told the BBC that he believed governments would be unlikely to impose a blanket ban again.
"I think if they'd sent up planes immediately to see whether the ash was actually too dangerous to fly through or to look for corridors where it wasn't very thick, I think that we would have been back flying a lot sooner," he said.
The bans were imposed because volcanic ash - a mixture of glass, sand and rock particles - can seriously damage jet engines.
The European decision to partially reopen airspace did not come until the fifth day of the crisis, when transport ministers met by teleconference.
'No over-reaction'
The UK Transport Secretary, Lord Adonis, said international safety regulators had been too cautious in their handling of the crisis.
But Henri Gaudru, the president of the European Volcanological Society, said there had been few options beyond the flight ban.
"This was not an overreaction. We... do not know enough about these clouds and what can happen to planes flying into them," he told a news conference in Geneva.
Earlier, the European air traffic agency Eurocontrol said it expected "almost 100%" of flights to operate in the continent on Thursday.
However, a fresh volcanic ash alert led Australian airline Qantas to cancel one flight out of London and delay another for 11 hours until early on Thursday, infuriating passengers.
At London's Heathrow Airport, Europe's busiest, traffic ran at 90% normal service on Wednesday. Many night flights are being allowed temporarily to help clear the backlog of stranded passengers.
Transatlantic services have returned to their normal level, with 338 flights arriving in Europe on Wednesday, according to air traffic agency Eurocontrol.
German airline Lufthansa said it would fly at full capacity by operating about 1,800 flights on Thursday, up from about 700 on Wednesday.
Air France said its long-haul flights were now departing as normal.
Denmark, Norway and Sweden have lifted their no-fly bans, but some airspace restrictions remain over Finland and some remote Scottish isles.
In Iceland, the Eyjafjallajokull volcano continues to erupt, but it is no longer spewing out ash into the atmosphere.
"There is much, much, less ash production and the plume is low," Gudrun Nina Petersen of the Icelandic Met Office said.
