LONDON - Airlines moaned in Europe and passengers groaned as ash from an erupting Icelandic volcano brought days of uncertainty to thousands of air travelers.

Hundreds of flights were canceled Tuesday as winds blew a cloud of ash from the Grimsvotn volcano over to Scotland. Experts say particles in the ash can stall jet engines and sandblast planes' windows.

The only comfort for frustrated passengers and airlines was that officials in Iceland said the amount of ash being released by the volcano was decreasing, and officials didn't expect the disruption to be as bad as one last year, when millions were stranded after the eruption of the Eyjafjallajokull volcano.

Nonetheless, British authorities said concentrations of ash in the skies over Scotland were high.

"All the data we are receiving confirms our forecasts, that there is high-density ash over Scotland," said Barry Grommett, spokesman for Britain's weather agency.

The Irish budget airline Ryanair challenged the results, saying it had sent its own airplane into Scottish airspace and found no ash.

"Exactly as we predicted, we encountered absolutely no problems," Ryanair's chief executive, Michael O'Leary, said. "There's no cloud over Scotland. There's no dusting of ash on the airframe or the wings. The airspace over Scotland should never have been restricted in the first place."

Nonetheless, Ryanair was forced by Irish authorities to cancel all 68 of its flights in and out of Scotland for the rest of Tuesday. Seven other airlines - most of them regional carriers - also grounded their Scottish flights.

In Edinburgh, hundreds of passengers faced possible overnight stays in the city.

"I've been told I'll get home [Wednesday], but who knows," said Kgeld Westh, an architect from Copenhagen, Denmark, who headed to a Edinburgh hotel.

The Grimsvotn volcano began erupting Saturday, sending clouds of ash into the air. The amount of ash spewing from the volcano tapered off dramatically Tuesday, however, said Elin Jonasdottir, a forecaster at Iceland's meteorological office.