BRUSSELS (Reuters) - European officials said they hoped half of flights would operate across the continent on Monday as they sought to ease four days of airline paralysis caused by a sprawling ash cloud from an Icelandic volcano.

The closure of most of Europe’s airspace except for the eastern and southern rims has cost airlines and airports hundreds of millions of dollars, and they called on Sunday for a review of the restrictions. The closure has also stranded millions of passengers and hurt exporters.

The Dutch airline KLM, which flew several test flights, said most European airspace was safe despite the plume of ash, and dispatched two commercial freight flights to Asia on Sunday evening.

European Union Transport Commissioner Siim Kallas said he hoped 50 percent of European airspace would be risk-free on Monday, adding the current situation was not sustainable. “We cannot wait until the ash flows just disappear,” he said.

“The forecast is that there will be half of flights possibly operating tomorrow,” said Spanish Secretary of State for European Union affairs Diego Lopez Garrido. “It will be difficult; that’s why we have to coordinate,” he told reporters after a meeting at European aviation control agency Eurocontrol.

Italy and Austria said they would reopen affected airports on Monday.

Only 5,000 flights took place in European airspace on Sunday, compared with 24,000 normally, Eurocontrol said. It said 63,000 flights had been canceled since Thursday.

Iceland’s Meteorological Office said tremors from the volcano had grown more intense but that the column of ash rising from it had eased back to 4-5 km (2.5-3 miles) from as high as 11 km when it began erupting on Wednesday from below the Eyjafjallajokull glacier.

Volcanic ash is abrasive and can strip off aerodynamic surfaces and paralyze an aircraft engine. Aircraft electronics and windshields can also be damaged.

HOPE OF RELIEF

Weekend test flights with empty planes offered some hope.

“Lufthansa made 11 flights, KLM nine, Air France seven and the results show no impact in the area ... No impact coming from the ash cloud,” Garrido said.

KLM said its inspections showed no damage to engines or evidence of dangerous ash concentrations. Its chief executive, Peter Hartman, was quoted by Dutch media as saying European airspace was safe “with the exception of an area in the north between Iceland and Russia.”

Airline and airport groups called for the flight restrictions to be reassessed.

“The concentration of ash particles in the atmosphere is in all likelihood so small that it poses no threat to air transport,” the association of Dutch pilots said.

British Airways chief executive Willie Walsh flew in a test flight over Ireland which the airline said encountered no difficulties. But BA and Irish Aer Lingus canceled all their flights for Monday, and Ireland’s Ryanair canceled flights to and from northern Europe until Wednesday.

The Spanish EU presidency called a video conference of EU transport ministers for Monday.

“We can examine the results of the test flights and look and see whether there is any updating of the regulatory structure which might make it possible for flights to take place despite the presence of the ash cloud,” British Transport Minister Andrew Adonis told BBC television.

Dutch state broadcaster NOS reported Transport Minister Camiel Eurlings as saying Europe’s response to the ash cloud had been too severe, and that the United States did not completely close its airspace in response to similar eruptions.

Brian Flynn of Eurocontrol said the International Civil Aviation Organization published rules that needed to be adhered to worldwide, and guidelines to interpret at continental level.

“One could say that the guidelines are interpreted slightly more rigorously in Europe than in the United States, when it comes to responsibilities of air traffic control agencies and pilots,” he told Reuters.

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The clampdown poses a growing problem for airlines, estimated to be losing $200 million a day, and for millions of travelers stranded worldwide.

Weather experts said wind patterns meant the ash plume was not likely to move far until later in the week.

It was expected to become more concentrated Tuesday into Wednesday, posing a greater threat to air travel, but narrowing to affect a smaller area. A shift in jet stream winds from Thursday could flush it out of most of Europe.

“It’s like a spray can of ash coming from Iceland,” U.S.- based forecaster AccuWeather said. “As with a spray can, the plume of ash is not uniform. It becomes deformed and spreads out in different directions the farther from the source it gets.”

For some businesses dependent on fast air freight, the impact has been immediate.

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Kenya’s flower exporters said they were already losing up to $2 million a day. Kenya accounts for about a third of flower imports into the European Union.

The disruption is the worst since the September 11 attacks on New York and Washington in 2001, when U.S. airspace was closed for three days.

President Barack Obama, German Chancellor Angela Merkel and others canceled trips to Poland for the funeral of President Lech Kaczynski.

For travelers, businesses and financial markets the biggest problem remains the sheer unpredictability of the situation.

ECONOMIC HAZARDS

Economists say they stand by their predictions for European growth, hoping normal service can resume this week.

But if European airspace were closed for months, one economist estimated lost travel and tourism revenue alone could knock 1-2 percentage points off regional growth. European growth had been predicted at 1-1.5 percent for 2010.

“That would mean a lot of European countries wouldn’t get any growth this year,” said Vanessa Rossi, senior economic fellow at Chatham House. “It would literally stifle the recovery. But the problem is it is incredibly hard to predict what will happen. Even the geologists can’t tell us.”

Disruption spread to Asia, where dozens of Europe-bound flights were canceled and hotels from Beijing to Singapore strained to accommodate stranded passengers.

More than four in five flights by U.S. airlines to and from Europe were canceled on Saturday. Freight company FedEx Corp said more than 100 FedEx Express flights headed to Europe had been rerouted, diverted or canceled over 72 hours.

Russian airports remained open, routing planes to North America over the North Pole to avoid the cloud.