Ash from Iceland's spewing volcano halted air traffic across a wide swathe of Europe Thursday, grounding planes on a scale not seen since the 9/11 terror attacks. Thousands of flights were canceled, tens of thousands of passengers were stranded and officials said it was not clear when it would be safe enough to fly again.

As of 7:30 Thursday morning, two flights at Sea-Tac were cancelled, British Airways flight 49 scheduled to arrive from London-Heathrow at 4:45 p.m. and British Airways flight 48 scheduled to depart at 6:45 p.m.

One scientist in Iceland said the volcanic ash - and disruptions in air travel - could continue for days or even weeks.

Authorities stopped all flights over Britain, Ireland and the Nordic countries. The shutdown closed London's five major airports including Heathrow, Europe's busiest. The major trans-Atlantic hub handles over 1,200 flights and 180,000 passengers per day.

With the cloud drifting south and east across Britain, the country's air traffic service banned all non-emergency flights until at least 11p.m. Thursday night, our time.

Irish authorities closed their air space for at least eight hours, and aviation authorities in Denmark, Norway, Sweden and Finland took similar precautions.

The volcano's smoke and ash poses a threat to aircraft because it can affect visibility, and microscopic debris can get sucked into airplane engines and can cause them to shut down. Health officials said the plume, which rose to between 20,000 feet and 36,000 feet (6,000 meters and 11,000 meters), posed no threat to human health.

Airlines in the United States were canceling some flights to Europe and delaying others.

Federal Aviation Administration spokeswoman Laura Brown said her agency was working with airlines to try to reroute some flights around the ash cloud, which lies above the Atlantic Ocean close to the flight paths for most routes from the U.S. east coast to Europe.