Three of Europe's busiest airports reopened Monday afternoon after a dense volcanic ash cloud from Iceland dissipated and a no-fly zone was lifted. Up to 1,000 flights in Europe were affected by the closures.

Flights were landing and taking off from London's Heathrow and Gatwick airports and Amsterdam's Schiphol Airport, but all three warned travelers it would take time for airlines to clear the backlog of delayed flights and to contact their airlines before coming to the airport.

Eurocontrol, the continent's air traffic control agency, said 28,000 flights were expected Monday in Europe, about 1,000 less than normal, mainly due to the disruptions in Britain and the Netherlands.

Icelandic civil protection official Agust Gunnar Gylfason said the ash cloud was traveling to the north, forcing airports in Keflavik and Reykjavik to close. He said seismic activity at the volcano was unchanged.

All British airspace was open except for smaller airports on remote Scottish islands.

Dublin's international airport reopened at noon. The only Irish airport still closed was Donegal, in the northwest.

Airspace over the North Sea was closed until midnight GMT (8 p.m. EDT), forcing airplanes to fly around it, and Faeroe Island airports were closed.

British Airways chief executive Willie Walsh -- facing a fresh wave of strikes by cabin crews on Tuesday -- called the latest airspace closures "a gross overreaction to a very minor risk."

"I am very concerned that we have decisions on opening and closing of airports based on a theoretical model," he said. "There was no evidence of ash in the skies over London today yet Heathrow was closed."

Aviation officials have defended the decision to impose the no-fly zone, saying airline representatives and engine manufacturers last week had agreed to find a way to ensure planes could fly safely in the volcanic ash.

Germany sent up two test flights Sunday to measure the ash cloud, but there was no word yet on the results of those tests. Still, Germany said Monday the latest ash cloud should not affect its airports.

"At this time, the concentration of ash above German air space is so low that there are no reductions in air traffic," German Air Traffic Controllers said.

Ash can clog jet engines. The April 14 eruption at Iceland's Eyjafjallajokul volcano forced most countries in northern Europe to shut their airspace between April 15-20, grounding more than 100,000 flights and an estimated 10 million travelers worldwide. The shutdown cost airlines more than $2 billion.

Airlines complained bitterly over the airspace closures last month, calling them an overreaction. The European air safety agency last week proposed drastically narrowing the continent's no-fly zone because of volcanic ash to one similar to that used in the U.S. The proposal still must be approved.

Eurostar added four extra trains Monday -- an additional 3,500 seats -- between London and Paris to help travelers cope with the airport closures.

Eyjafjallajokul (pronounced ay-yah-FYAH-lah-yer-kuhl) erupted in April for the first time in nearly two centuries. During its last eruption, starting in 1821, its emissions rumbled on for two years.