"There are no standards at the moment on the concentration of ash that could affect" airplane engines, Roberto Kobeh Gonzalez told reporters in response to questions on whether it was safe to fly, as Iceland's Eyjafjoell volcano spewed more ash.

"We cannot invent the standards. This is something we have to work with the industry," the Mexican head of the Montreal-based organization added, pointing out that the answer had to come from scientists.

ICAO Secretary General Raymond Benjamin agreed that there was no easy answer.

"If you ask a manufacturer of engines: 'What is the concentration of ash that your engine can sustain?,' he will not answer you because it's a matter of liability," he said. "No one will fly if there is a risk."

A giant volcanic ash cloud has snarled air traffic to and from Europe in recent days, stranding thousands of passengers.

After days of aviation chaos, flights have now resumed, though a massive backlog of travellers remains.

Kobeh meanwhile rejected suggestions that ICAO had failed to coordinate the world response to the crisis.

"The states are responsible for the safety (of air flights) following the standards of ICAO," he said. "We do not have a coordinating role."

Benjamin said a meeting bringing together the airline industry, aircraft manufacturers, the International Air Transport Association, governments and scientists would be convened soon to start focusing on the issue.

Both officials stressed that it was now safe to fly from North America to Europe.

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