European Union transport ministers are due to meet on Monday by teleconference to decide whether to reopen air space - amidst growing claims by airlines that the volcanic ash cloud from Iceland poses no safety threat to jet engines.
"This is not sustainable, we cannot go ahead and wait until the ash cloud will disappear," the European Commissioner for Transport, Siim Kallas, told journalists late Sunday, dpa reported.
He was speaking in Brussels after talks with officials from Eurocontrol, the European air safety body, and a representative from the Spanish government, which holds the EU's rotating presidency.
On Sunday 84 per cent of European flights were grounded, according to Eurocontrol forecasts. But Spain's EU affairs minister, Diego Lopez Garrido, said the situation was expected to ease on Monday.
"The forecast is that there will be half of the flights possibly operating in Europe," he said, explaining that the ash cloud was expected to be "moving slightly to the north east," leaving "half of EU territory" clear of any obstruction.
Garrido said technical talks with the European Commission and Eurocontrol would continue Monday morning, so that EU transport ministers would later be able to take a joint "European decision ... based on technical advise and (considering) safety as a priority."
The Spanish official said "the aim (was) to open progressively the European (air) space," after test flights operated over the weekend by Lufthansa, KlM and Air France showed there was "no impact" from the volcanic ash.
However, Garrido rejected claims that air traffic authorities overreacted by bringing most of Europe's air traffic to a standstill, causing misery for millions of passengers and leaving airlines to face losses running into hundreds of millions of dollars.
"They were not arbitrary decisions, but based on scientific advice," the Spanish minister insisted.