European transport ministers are to hold emergency talks by video conference on easing the air travel crisis caused by a volcanic ash cloud in Europe's skies.
More than 6.8 million passengers have been affected as the crisis enters its fifth day.
Airports and airlines are questioning the need for curbs said to be costing airlines hundreds of millions of dollars a day.
The International Air Transport Association has called for a move away from blanket closures towards ways of flexibly opening air space, step by step.
The enormous shroud of fine mineral dust particles now stretches from the Arctic Circle in the north to the French Mediterranean coast in the south, and from Spain into Russia.
Airspace remains closed, or partially closed, in more than 20
countries. Altogether, 63,000 flights have been cancelled, paralysing more than 300 airports in Europe.
There were only 5,000 flights in European air space on Sunday, compared with 24,000 normally.
Many airports and airlines are becoming impatient with restrictions and have called for an urgent review of the measures.
Most airlines say services to and from Europe won't resume until mid-week at the earliest.
Air New Zealand says it will not resume flights to Europe before Tuesday morning and Singapore Airlines says the same.
But Qantas says its flights to Europe won't resume at least till Wednesday. Emirates says its passengers will also have to wait until Wednesday to fly to Europe.
Spanish air space has reopened after a brief closure and Spain has suggested its airports could be used an an entry point to Europe.
Britons stranded in Asia would fly into Spain then continue their journey by train, boat or coach.
The prime ministers of Britain and Spain have spoken about the plan to establish a Spanish hub, which could also involve using navy and merchant ships to ferry Britons home from Spain by sea.
As well as Spain, flights are now operating in Greece, Lithuania, Portugal, Russia, Turkey and Ukraine.
Bodies representing airports and airlines have called for restrictions in northern Europe to be reviewed. Lufthansa, Air France, KLM and British Airways have carried out test flights.
German carriers Lufthansa and Air Berlin have expressed anger that the decision to ground flights appears to have been taken solely on the basis of a computer simulation.
The head of Air Berlin said not a single air balloon had been sent up to measure how much ash was actually in the air.
The two carriers, as well as Air France and the Dutch airline KLM, said they had found no problems with test flights they had carried out.
Britain's transport minister, Andrew Adonis, says there is strong safety advice that passenger flights cannot take place while the ash cloud is present, but says it may be possible to update the regulatory structure so services can resume.
