The UK is experiencing its fourth day as a virtual no-fly zone due to volcanic ash drifting from Iceland, leaving thousands of Britons stranded.
Flight restrictions have been extended until at least 0700 BST on Monday and forecasters say the ash cloud could remain over the UK for many more days.
Several countries including the UK are carrying out test flights.
Prime Minister Gordon Brown has called a top-level ministerial meeting to discuss the situation.
A Downing Street spokesman said: "The meeting will cover a number of issues, including the assistance being provided to those Britons who have been unable to travel home, and the implications for industry.
"They will also look at what more can be done on a European level."
On Monday, EU transport ministers are meeting to consider whether flights could resume even if volcanic ash remained in the atmosphere.
Dutch airline KLM and German airline Lufthansa have carried out test flights in their countries' airspace to see if it is safe for planes to fly.
Austria, Belgium, Bulgaria, Czech Republic, Denmark, Finland, France, Germany, Ireland, Latvia, Luxembourg, Netherlands, Poland, Slovakia, Sweden, Switzerland, UK
Italy (northern airspace closed until Monday)
Norway (limited flights in north)
Spain (northern airports closed)
Greece, Portugal, Russia, Turkey, Ukraine
KLM, which is inspecting test plane engines for possible damage with a view to restarting operations, said its aircraft had been able to fly at normal operating altitude of 13km (8 miles) over Dutch skies and no problems had been reported.
"We have found nothing unusual, neither during the flight, nor during the first inspection on the ground," said KLM chief executive Peter Hartman, who took part in his airline's test.
Lufthansa said it flew 10 planes from Frankfurt to Munich at heights of up to 8km (5 miles).
Air France said it had successfully carried out a test flight from Paris to Toulouse.
Germany has opened six airports for flights heading east until 1900 BST, and several will remain open in southern France.
Planes were first grounded in the UK at midday on Thursday amid fears that particles in the ash cloud generated by the volcanic eruption could cause engines to shut down.
Transport secretary Lord Adonis said further test flights would take place in the UK to help understand the extent of the impact of the ash cloud.
He said: "I wish to establish, as a matter of urgency, whether some safe flight paths can be identified and opened up to flights within the area affected by ash."
Meanwhile, Brian Flynn, head of operations at Eurocontrol, the organisation in charge of air safety in Europe, denied aviation authorities were being over cautious.
He said: "The accepted methodology that we have in Europe - the guidelines of the International Civil Aviation Organisation - are the guidelines that we are using, and that is that any risk of an aircraft penetrating an area that could have volcanic ash in it could have extreme safety consequences."
Mr Flynn said the "over-riding objective of protecting the travelling public" meant exceptional measures had to be taken.
AT THE SCENE
Lorna Gordon, BBC News, Iceland
In some small areas the volcanic fallout has been significant. It is clogging car engines, turning grass grey and reducing visibility to just a few metres.
The police say driving conditions can be very difficult in these places. I heard one tale recounting that the moment you drive into the ash cloud it can feel as if you are driving into a wall.
The affected area is remote with only a few hundred people, most of them living in isolated homes and many of them farmers. They have been advised to stay inside with the windows and doors shut and if they do venture out to wear goggles and a mask.
Despite the hazards the volcano and its column of smoke are drawing visitors. They are also triggering lightning. The authorities are having to remind people they should not consider the volcano a tourist attraction. In fact, they have closed the country's ring road in the area affected to all but locals and the emergency services.
The International Air Transport Association (IATA) has warned airlines would lose at least £130m a day in revenues during the disruption.
BBC business editor Robert Peston said the disruption risked becoming a "major business and economic disaster".
He said a number of European airlines were facing financial difficulties.
Our correspondent said: "If [the disruption] goes on many days longer, a number of European airlines will run into financial difficulties and may need bailing out by governments - or so I am told by senior airline figures."
Meanwhile, the Ministry of Defence is considering plans to fly troops wounded in Afghanistan to coalition partner countries such as Germany for treatment if UK airspace remains closed.
Under normal circumstances they would be flown home and treated at Selly Oak Hospital in Birmingham.
The disruption has affected hundreds of thousands of travellers since Wednesday, when the Eyjafjallajoekull volcano began erupting.
Cassandra Williams, who had been expecting to start her new job as a head teacher in Norfolk on Monday, is stuck in Hong Kong.
She said: "There are lots of children meant to be taking GCSEs and A-levels and they are very worried about when they might be able to fly to get home."
Karen Abbott, from Southampton, is stuck in Singapore with her husband and daughter, who has diabetes and is insulin-dependent. She said the family's medication stock was running out.
Anyone concerned about the safety of a British national stranded abroad can call a Foreign Office helpline on 020 7008 0000, or visit its website at www.fco.gov.uk.
Stranded Britons should contact their local embassy, high commission or consulate.
In other developments:
• British Airways cancelled all long and short-haul flights in and out of the UK on Monday
• An attempt by TV presenter Dan Snow to rescue people stranded in France using five rigid inflatable boats was halted by French authorities
• The impact is likely to exceed the airspace shutdown after the 11 September 2001 attacks, according to the International Civil Aviation Organisation
• P&O ferry crossings between Portsmouth and Bilbao are fully booked until Wednesday, as are those from Hull to Zeebrugge and Rotterdam over the weekend
• The Prince of Wales and Foreign Secretary David Miliband cancelled their journey to the funeral of Polish President Lech Kaczynski
• Royal Mail said international mail due to leave or enter the UK was likely to be delayed. Mail to much of the US was being routed via southern Europe