Volcanic ash from Iceland snarled air traffic across Europe for a second straight day Friday, causing the cancellation of some 17,000 flights, according to the intergovernmental body that manages European air travel, according to CNN.

Eurocontrol said it expected around 11,000 flights to take place Friday, in contrast to the normal 28,000. The impact will last at least another 24 hours, Eurocontrol said Friday morning.

The ash has forced the closure of some of Europe`s busiest airports including London`s Heathrow and Schiphol in Amsterdam.

Airspace over much of the United Kingdom was closed to all flights except emergencies at least until 1 a.m. Saturday (8 p.m. ET Friday), Britain`s air traffic authority, NATS, said.

Selected flights were being allowed in Northern Ireland and Scotland, and some North Atlantic traffic may be allowed to those areas, it said.

NATS called the situation "dynamic and subject to change."

Airspace was also closed over Belgium, Finland, Lithuania, Estonia, and the Netherlands, and airspace was partially closed in France, Denmark, Germany, Poland, Sweden, Norway, and Ireland, Eurocontrol and local authorities said.

Norway`s airspace closure included cancellation of helicopter flights to off-shore oil installations.

Germany`s closure included airports in Berlin, Frankfurt, and Duesseldorf, Eurocontrol said.

Warsaw airport in Poland was closed, which may affect plans for world leaders to attend the scheduled funeral of Polish President Lech Kaczynski.

The U.S. Air Force said two bases in England, RAF Mildenhall and RAF Lakenheath, will be shut down for at least two days. That meant dozens of U.S. Air Force F-15s and other fighter jets and tankers are not flying, and flights to Iraq and Afghanistan flying through that airspace are being diverted to other routes.

English Channel ferry operators and Eurostar, which runs high-speed trains between London, Paris, and Brussels, said they were experiencing an extremely high demand as passengers seek other ways of getting to their destinations.

Delta Air Lines canceled 65 international flights from its U.S. hubs scheduled over Thursday night and Friday morning in response to the ash cloud. The decision affected flights to Amsterdam; London; Shannon, Ireland; and Brussels in Europe and to Mumbai in India. Whether the airline resumes flights to such destinations will be determined later Friday, Delta said.

Air China announced the cancellation of most of its Friday flights to Europe.

Hong Kong-based Cathay Pacific also canceled or delayed flights from Hong Kong to London, Paris, Frankfurt and Amsterdam.

Volcanic ash can cause jet engines to shut down.

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, 30-35,000 feet, and you then need the upper level winds to blow the ash and disperse it out of the airspace," he told CNN.

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 U.K. airspace," he said.

The closures came after an eruption under an Icelandic glacier early Wednesday. The eruption under the Eyjafjallajokull glacier -- the latest in a series that began on March 20 -- blew a hole in the mass of ice and created a cloud of smoke and ash that went high into the air.

The volcano was still erupting Friday and producing a lot of volcanic ash, a spokesman for Iceland`s Department of Civil Protection and Emergency Management told CNN.

"The volcanic ash particles are very fine and have been compared to flour and sugar in size," the department said Friday.

"The flourine content of the ash is high and presents a health risk to livestock in areas where the ash layer is more than 1 centimeter thick."

Dust and odors were detected Thursday in the Northern Isles in Scotland, the British weather service, the Met Office, said Friday. The Scottish Environment Protection Agency is now analyzing the dust.

The Met Office said the volcano is spewing ash in "pulses," not a steady plume.

An emergency evacuation affecting 800 people was conducted near the volcano due to flash flooding from glacier waters, according to Rognvaldur Olafsson of Iceland`s civil protection agency.

The same people had fled the area the night before and allowed to return to the area early Thursday.

The ash wasn`t necessarily visible in the air, but Manchester Airport spokesman Russell Craig said it can still pose a threat to aircraft.

"If you think about the way an aircraft engine works, it sucks in air, it compresses it, forces it out on the other side. That creates thrust," Craig told CNN.

"If that air were mixed with ash, it can cause engine failure and electrical difficulties with an aircraft. It`s happened before, and the aircraft didn`t come out the other end in one piece."

Capt. Bob Jones of the Civil Aviation Authority said Britain will not reopen its airspace until the threat is over.