The unprecedented four-day shutdown of European airspace appeared set to continue wreaking havoc, as the volcano near the Eyjafjallajoekull glacier in southern Iceland went on spewing ash and many countries once again extended their flight bans.

Every day of the widespread travel ban is costing the airline branch 150 million euros ($202 million), the International Air Transport Association (IATA) estimated.

Some airline officials have started to question whether the bans that started Thursday were truly needed.

Representatives of Germany's two largest airlines, Lufthansa and Air Berlin, criticised their country's airspace shutdown, saying the spread of the volcanic ash had only been computed through a simulation in London and not studied through actual atmospheric measurements.

Both airlines flew planes without passengers Saturday to have aircraft where they were needed once the flight ban was lifted.

A spokesman for Lufthansa, which flew 10 planes up to a height of 8,000 metres, said that not even "the slightest scratch" could be found on the aircraft afterwards.

Airlines had feared that the powdery, abrasive ash could get sucked into planes' jet engines, causing them to cut out.

French and Dutch airlines also conducted test flights, some of them Sunday. The French environmental minister called for a telephone conference of European Union transport ministers to be held Monday to discuss further tests.

European Commission president Jose Manuel Barroso in turn announced Sunday that a task force is to be set up in Brussels to evaluate the cost to the EU's economy of the volcanic ash cloud.

In the meantime, hundreds of thousands of air passengers remained stranded throughout Europe, with many resorting to rental cars, trains and buses as an alternate way to their destination.

The European air traffic safety body Eurocontrol expected 84 percent of Europe's scheduled flights to be grounded Sunday, with only 4,000 out of the 25,000 flight that are normal for a Sunday able to take off or land.

The figure represents the highest number of flights scrapped since travel disruptions started Thursday, with air travel restricted in a total of 24 countries.

The flight bans were also being felt far beyond Europe, with Australia's biggest airline Qantas for instance announcing that it would not attempt to fly to Europe until at least Tuesday.

In Europe, meanwhile, commercial flights were blocked in Germany until at least late Sunday. Britain, France, northern Italy, Finland, Denmark, the Czech Republic, Austria and Hungary extended their flight bans into Monday morning.

Some international airlines, such as Thai Airways International, resorted to adding more flights to southern European cities such as Rome and Madrid in an attempt to facilitate return trips for their stranded passengers.

But as the rolling cloud of ash traveled further south with the jet stream, the airport closures continued.

Spain Sunday announced that it shut down 13 northern airports, including hubs in Barcelona and Mallorca. Planes continued to take off in other parts of that country, along with Portugal, the Balkans, Italy, Bulgaria, Greece and Turkey, Eurocontrol said.

Even as they grounded planes on their territory, some countries were still allowing transit flights to cross their airspace without landing. But Switzerland and Poland required those flights to travel at an altitude of at least 11 and six km, respectively.

The continuing flight ban in Poland forced many world leaders to scrap their plans to attend Sunday's funeral of President Lech Kaczynski and his wife, who died in an April 10 plane crash.

Czech President Vaclav Klaus travelled to the funeral by train and car.
German Chancellor Angela Merkel, meanwhile, flew from the US to Lisbon and then Rome, before embarking on a road trip through Austria and Germany - which ended up being delayed by a blown bus tyre.

Meteorologists and volcanologists in Iceland estimate that the volcano will continue to spew ash and steam for "definitely days, but maybe also weeks and months".
Experts estimate that it will take at least a week for air travel to normalise once flight bans are lifted.