(CNN) -- Ash from an Icelandic volcano is continuing to affect European flights on Monday, delaying transatlantic aircraft and threatening flights over parts of Ireland and Portugal.
Eurocontrol, the agency that manages European air travel, said: "During the afternoon, areas of higher ash concentration could move in a north-easterly direction from the Atlantic into the Iberian Peninsula."
Ryanair, the budget airline, canceled 18 flights on Monday as it expected the airspace surrounding Kerry airport in the southwest of Ireland and Faro airport in Portugal to be affected by the ash cloud.
Ryanair said in a statement on its Web site: "Passengers are strongly recommended to check their flight status before they go to the airport."
Eurocontrol said much of the high-concentration ash cloud over continental Europe had dispersed and that despite the delays, all European airports were open on Monday afternoon.
The UK Civil Aviation Authority said that transatlantic flights were still rerouting around ash that was lying between 20,000 to 35,000 feet in the atmosphere.
A CAA spokesman told CNN: "The ash cloud is slowly disappearing, it's descending and dispersing, there isn't one big cloud... it's ash in the air over a very large area. Transatlantic flights are rerouting around it, causing delays."
Eurocontrol said it expected approximately 28,500 flights within the European area, which is about 500 below average for a Monday at this time of year.
Italian airports in Milan, Pisa and Florence as well as six airports in Scotland were closed over the weekend because of the ash cloud.
Volcanic ash can be a serious hazard to aircraft, reducing visibility, damaging flight controls and ultimately causing jet engines to fail.
The problems began in mid-April, when the volcano beneath the Eyjafjallajokull glacier in southern Iceland erupted and sent a cloud of ash into the atmosphere, closing most of Europe's airspace for six days.