Belgium, Italy and Ireland are among the other countries to allow a gradual resumption of flights. In northern Europe, however, it was a different story. Sweden kept its airspace closed while Denmark said it would temporarily reopen most of its airspace and airports. Meanwhile, Finland's airports will remain closed until 0600 GMT Wednesday.

Airlines elsewhere also started to resume flights. Air China said it had resumed routes between Beijing and destinations including Moscow, Stockholm and Rome.

Although these new developements have brought great relief to stranded passengers, officials have warned that considering the huge backlog of travelers, airlines faced the enormous task of flying all of them back home, a challenge that could take days or even weeks.

Passengers with current tickets were being given priority; those who had been stranded for days were told to either buy a new ticket or take their chances using the old one -- a wait that could be days or weeks for the next available seat.

To add to these worries, scientists now fear possible new, larger eruptions that may cause even worse travel chaos than the current one.

As Icelandic police report "still considerable activity" at the Eyjafjoell volcano, scientists say tremors there could trigger an even more dangerous eruption at the nearby Katla volcano. A Katla eruption would be 10 times stronger and shoot higher and larger plumes of ash into the air.

The two volcanoes are situated side by side in southern Iceland, about 12 miles (20 km) apart and thought to be connected by a network of magma channels.

Last Wednesday's volcanic eruption has led to an almost complete shutdown of European airspace for the past four days, affecting nearly 7 million passengers and worsening the situation of the air industry, which is already struggling in the aftermath of the financial crisis.

Companies are losing 270 million U.S. dollars per day, according to the International Air Transport Association (IATA).

Although officials have justified the widespread airport closures by saying that airplane engines could become clogged up and stop working if they tried to fly through the ash, airlines have criticized the sweeping measures as unnecessary, saying they caused chaos and massive financial losses.

The International Civil Aviation Organization (ICAO) announced Tuesday it will start setting global standards for the concentration of ashes that could affect airplane engines. This was in reaction to accusations that the agency failed to coordinate global airlines' activities in the aftermath of the travel chaos after the volcanic eruption.

Raymond Benjamin, secretary-general of the U.N. agency ICAO, told reporters at the United Nations that ICAO convened a special meeting of its governing council on Monday to discuss ash standards following this global disruption to air travel.

He said the council decided to convene a group including representatives from industry, manufacturers, governments, scientists, and IATA "to start working on these standards."