A cloud of ash from a volcano in Iceland swept toward mainland Europe on Thursday, forcing up to 6,000 flight cancellations across the continent, according to the intergovernmental body that manages European air travel

Thousands of flights were affected as some of Europe’s busiest airports closed, including London’s Heathrow; Amsterdam, Netherlands’ Schiphol; and Paris, France’s Charles de Gaulle.

Airspace over the United Kingdom was closed to all flights except emergencies at least until 1 p.m. local time (8 a.m. ET) Friday, Britain’s Civil Aviation Authority said. The agency was due to review the situation Thursday evening.

France, Germany, Ireland, Sweden, Norway, Belgium, Denmark and the Netherlands also announced the complete or partial closure of their airspace, authorities in each country said.

Norway also closed its ocean territory and canceled helicopter flights to offshore oil installations, according to Avinor, the Norwegian agency responsible for the country’s airport network.

Germany closed the Berlin airport as well as airspace over the city, an airport spokesman said.

Delta Airlines canceled 65 international flights from its U.S. hubs scheduled over Thursday night and Friday morning. The decision affects flights to Amsterdam; London; Dublin, Ireland; Brussels and Mumbai, India. Whether the airline resumes flights to such destinations will be determined Friday, Delta said.

It’s hard to predict how long it will be before air travel can resume, said Matthew Watson, a geophysicist, at England’s Bristol University.

“You really need two things to happen: You need the volcano to stop emplacing ash to the altitude that commercial aircraft fly at, 30,000 to 35,000 feet, and you then need the upper-level winds to blow the ash and disperse it out of the air space,” he said.

How long that will take “depends very much on the volcano. If this is it and it’s stopped right now and it doesn’t do anything else … I imagine you are looking at 24 to 48 hours to clear UK air space,” he said.

But the volcano was continuing to erupt and spew ash as of 5:30 p.m. local time (1:30 p.m. ET) Thursday, Icelandic Foreign Ministry representative Urdur Gunnarsdottir said.

France closed eight airports in the north of the country as of 5 p.m. local time (11 a.m. ET) and is set to close another 16, including Charles de Gaulle, at 11 p.m. local time (5 p.m. ET).

Many airports were already shut and flights were grounded across the United Kingdom on Thursday because of the ash, which came after an eruption under an Icelandic glacier early Wednesday, airport authorities said.

The eruption — the latest in a series that began March 20 — blew a hole in the mass of ice and created a cloud of smoke and ash that went high into the air.

Flights to the United Kingdom from Japan, Hong Kong, India and Australia were affected, and Etihad Airways announced that five flights between Abu Dhabi and England were canceled Thursday.

(Read More: CNN.com)