Major European airlines sent test flights into the sky Sunday, and their pilots reported no damage from volcanic ash hovering above the continent.

Now the airlines are pressuring European governments to ease the ban on air travel. Air traffic has been frozen the past four days, due to ash spewing from a volcano in Iceland.

The test flights, which took place without any passengers on board, show that the skies are safe, according to airline officials and some commercial pilots.

However, meteorologists have warned that much of Europe's airspace remains unstable. The ash being propelled from an Icelandic volcano is still capable of crippling jet engines, they say.

Air traffic may return to half of its usual volume on Monday if the cloud of ash begins to dissipate, according to European Union officials.

German authorities have already allowed some flights to take off.

But 80 per cent of the continent's airspace was closed for the fourth day straight on Sunday, said Brian Flynn, deputy head of operations for at air-traffic safety authority Eurocontrol.

"Today it has been, I would say, the worst situation so far," Flynn said.

Airports from Ireland to Bulgaria began shutting down on Thursday as a result of ash from the erupting volcano. The effects have rippled through airports around the world.

KLM Royal Dutch airlines, Lufthansa, Air France and a number of other regional airlines initiated test flights above Europe. No pilots reported problems, and the planes were closely inspected for damage after landing.

"We observed no irregularities either during the flight or during the initial inspection on the ground," said KLM Chief Executive Peter Hartman, who was aboard a Saturday flight.

Steven Verhagen, vice president of the Dutch Airline Pilots Association and a KLM pilot, said he would feel safe flying, and the association called for the immediate resumption of flights.

"With the weather we are encountering now -- clear blue skies and obviously no dense ash cloud to be seen, in our opinion there is absolutely no reason to worry about resuming flights," Verhagen said.

"We are asking the authorities to really have a good look at the situation, because 100 per cent safety does not exist," Verhagen said.

However, no consensus has emerged on when affected airports may be able to reopen, not to mention when air travel may return to normal in Europe. Aviation authorities in each country will have to decide when to reopen their airspace.

"Normally, a volcano spews out ash to begin with and then it changes into lava, but here it continues to spew out ash, because of the glacier," said Reynir Bodvarsson, director of Swedish National Seismic Network. "It is very special."

Still, EU officials said they were hopeful that weather forecasts for the next 24 hours.

"Probably tomorrow one half of EU territory will be influenced," said Diego Lopez Garrido, state secretary for EU affairs for Spain. "This means that half of the flights may be operating."

He did not elaborate on which flights might resume.

The closure of much of Europe's airspace is costing the aviation industry at least $200 million a day, according to the International Air Transport Association.

Develpments this weekend:

• Switzerland began allowing flights through its airspace Saturday but required aircraft to fly at a minimum of 11,000 metres.
• Germany loosened its air traffic restrictions and began allowing flights from Berlin, Hamburg and three other airports until Sunday evening. But the busy airports at Munich and Frankfurt remain closed, and Air Berlin and Lufthansa have cancelled all flights until at least Sunday night.
• Britain said its airports would remain closed Sunday, as did Ireland. Dutch authorities said airspace closures will remain in place but will allow ongoing test flights.
• France's northern airports will be closed until Monday morning.
• Airspace over Denmark, Finland and most of Sweden remained closed Sunday, while Norway lifted air travel bans over the central part of the country. Other parts of Norway, including the capital, Oslo, remained closed to air travel.
• Closures extended east to Turkey, and the transport ministry said Sunday afternoon that it had suspended flights to three northern cities until Monday morning.

By the end of the day Sunday, more than 63,000 flights were cancelled as a result of the ash cloud since April 15, according to European air navigation and Eurocontrol.

The travel restrictions prevented a number of world leaders, including Prime Minister Stephen Harper and U.S. President Barack Obama, from attending Sunday's funeral of Polish President Lech Kaczynski and his wife, Maria, in Krakow, Poland.

With files from The Associated Press