Several airports in Sweden, Norway and Finland, including in Finnish capital Helsinki, were closed on Thursday due to the return of volcanic ash, air traffic authorities said.
In addition to Helsinki, which was likely to reopen at 1200 GMT, the airports in Norway's second and fourth largest cities, Bergen and Stavanger, were shut.
The airport in Sweden's second largest city Gothenburg was also closed, but scheduled to open again at 2:00 pm (1200 GMT).
Norway, Sweden and Denmark had completely reopened their airspaces on Wednesday, but eastern bound winds brought volcanic ash from Iceland back into Swedish and Norwegian skies Thursday, authorities said.
In Finland, where most airports had opened for only a few hours on Wednesday, authorities were hoping to open the Helsinki-Vantaa airport at 3:00 pm (1200 GMT) in time to welcome Finnair flights from Japan and China.
"According to forecasts, the ash plume will remain over parts of Finland in the afternoon and restrict flight traffic beyond northern-most Finland," airport authority Finavia said in a statement, adding Finnish airspace could be used normally for overflights.
"Current forecasts indicate, however, that in the afternoon it could be possible to open the Helsinki-Vantaa, Lappeenranta, Mariehamn, Savonlinna and Turku airports," it added.
In Sweden, six airports remained closed at 1100 GMT, including several smaller airports in the northeast and southwest of the country that were closed late Wednesday, the LFV air traffic authority said in a statement.
"The situation has gotten worse," the Swedavia airport authority said in a separate statement. "The forecasts are more uncertain than they have been."
The airport in the southern city of Malmoe was open after having briefly closed for a few hours early Thursday.
In Norway, six airports were still closed early Thursday, air traffic authority Avinor said.
Helicopter flights to Norwegian offshore oil platforms were also halted.
The closures in Norway, Sweden and Finland occurred as nearly all European airspace was open for business Thursday with a normal quota of between 28,000 and 29,000 flights expected for the first time in a week, air traffic control coordinator Eurocontrol said.
Skies were opened in Denmark, including Scandinavia's largest airport in Copenhagen.
Denmark also said it had opened all airspace above Greenland, after closing the southern part of it Tuesday because of the ash cloud.
But Danish air traffic authority Naviair said it could reintroduce flying restrictions if the concentration of volcanic ash in Danish skies increased, a spokeswoman told AFP.