MADRID — Air traffic over southern Europe faced more disruption Monday from an ash cloud from an Icelandic volcano, even as clearer skies in some areas let flights resume after a weekend of cancellations.
European air traffic agency Eurocontrol warned that by Monday afternoon "areas of higher ash concentration could move in a north-easterly direction from the Atlantic into the Iberian Peninsula," causing fresh flight troubles in Spain and Portugal.
About 500 fewer flights would take to the skies in Europe on Monday because of the ash cloud, which would also force transatlantic planes to fly lengthy detours, Brussels-based Eurocontrol said in a statement.
Spain was forced to impose overflight restrictions on air traffic between 20,000 and 35,000 feet (6,000 metres and 10,600 metres), the country's aviation authority, AENA, said in a statement.
The overflight restrictions came after Spain earlier reported the reopening of all 19 airports that had been affected by the ash cloud over the weekend.
A spokeswoman for AENA said that some air corridors normally reserved for the military may now be used for civil aviation.
An extra air corridor should also be opened Monday by the air traffic control centre in the Canary Islands "in order to help air traffic between the Americas and Europe."
In neighbouring Portugal, airports at Lisbon and the northern city of Oporto reopened early Monday but by 1600 GMT some 320 flights had been suspended, civil aviation authority NAV said.
Faro airport was closed from Monday evening as the ash cloud moved over Portugal's southern Algarve region, but was due to reopen midday Tuesday.
All flights to the Azores and Madeira remained grounded from the weekend.
Lisbon airport is the arrival point for Pope Benedict XVI who is due to begin on Tuesday a four-day visit to the country.
Portuguese Catholic Church officials have said that if necessary there is a "Plan B" to ensure the pontiff's visit goes ahead as planned.
Air traffic over French territory was almost normal, authorities said, barring some flight delays because of route changes, with weather forecasts favourable for Tuesday.
The eruption of the Eyjafjoell volcano in Iceland, which caused travel chaos worldwide with the airspace closed over many European nations for a week last month, was again causing delays due to "significant re-routings" of transatlantic flights, Eurocontrol said.
"Areas of high ash concentration have dispersed overnight over continental Europe," it underlined, adding there were around 1,500 fewer flights than anticipated on Sunday.
After a weekend of air travel disruptions, airports also reopened Monday in Austria, Italy, Germany, Ireland, Spain and Scotland, except for the airport at Barra island in the west.
But transatlantic flights to the United States, Canada and the Caribbean were suffering delays -- some for more than five hours -- especially those departing from London's Gatwick airport.
Saudi Airlines said it had halted flights between the United States and Saudi Arabia because of the ash cloud, and some of its flights between Europe and North America were also being delayed and rerouted.
Iceland's volcano began erupting on April 14 and spewed a cloud of volcanic ash that drifted over much of Western Europe for a week, closing airspace and grounding planes for fear that the ash would damage aircraft engines with fatal results.
It was the biggest aerial shutdown in Europe since World War II, with more than 100,000 flights cancelled and eight million passengers affected.
Geologists in Iceland say that while the situation should improve over the next few days, the volcanic eruption is not about to end soon.
The ash output hit a secondary high last Thursday, spewing a plume as high as 10,000 metres into the air, but had since decreased, Magnus Tumi Gudmundsson of the University of Iceland told AFP.
Eyjafjoell was on Monday emitting about 50 tonnes of ash every second, down from between 300-400 tonnes per second on Thursday, the geologist said.
However, he added: "The eruption is ongoing.... We see no end in sight."
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