Denmark reopened airspace above 11 km altitude Monday morning.

"The latest forecasts show that there is no ash cloud above 10.8 km altitude, and therefore we have no reason to keep it closed," said Communication Director of Naviair Bo Pedersen.

But Pedersen added the ash cloud is observed drifting below the height. "There is ash cloud moving down to the lower altitude. So we have to keep shutting the airspace below 11 km altitude."

The Danish airspace has been closed since 6 p.m. Thursday, and during that time, no flights depart or land in Denmark.

On Sunday evening, Eurocontrol, the Brussels-based air traffic body, said that 19,000 out of 24,000 flights were canceled on the day.