Europe remained a virtual no-fly zone for a fourth straight day yesterday as airlines urged an immediate re-think of flight restrictions to ease pressure on nearly seven million passengers grounded by volcanic ash.

However, the European Union presidency offered a glimmer of hope for stranded passengers, saying half of the flights scheduled for today in Europe could go ahead.

Several European carriers launched test flights to challenge warnings that the volcanic ash cloud spreading across Europe from Iceland's Eyjafjoell volcano since Wednesday would destroy jet engines. Air France and KLM reported no problems.

A grouping of the continent's 36 major carriers called on governments for an "immediate reassessment" of the restrictions, saying they were having a "devastating impact" on the industry, and questioning whether they were proportionate.

"Airlines must be able to fly where it is safe to fly and make decisions accordingly. It is what our passengers demand of us," the Association of European Airlines said.

Meanwhile, KLM Royal Dutch Airlines declared European airspace "safe" after a series of test flights, as the Dutch traffic authority said KLM would be allowed to fly three cargo flights to Asia later yesterday.

Two of Europe's three biggest airports, London Heathrow and Paris-Charles de Gaulle, remained shut.

But German authorities authorised the other, Frankfurt, plus five other national airports, to resume a limited service for several hours yesterday, depending on the flight's direction.

Airports that had been closed in northern and eastern Spain, and airports in southern France, also reopened.

"Forecasts are that half of the flights planned (in Europe) could take place tomorrow," Spanish Secretary of State for European Affairs Diego Lopez Garrido told a press conference yesterday at the Eurocontrol flight coordinator.