Ash clouds from an erupted Icelandic volcano drifted to parts of Europe on Thursday, forcing air-traffic controllers to close airspace to commercial aircraft and compelling airlines to cancel thousands of flights.

Roughly 3,000 of Europe's 28,000 daily flights have been grounded, according to a spokeswoman for Eurocontrol, a 38-country agency in Brussels that coordinates European flights.

Flights in the U.K. and western Scandinavia have been most affected so far, the spokeswoman said. Netherlands and Belgium are being progressively closed as the cloud spreads east and south.

Volcanic ash represents a significant safety threat to aircraft, as particles reduce visibility and can even cause aircraft engines to shut down.