Half of all Europe's flights were back in the air Tuesday, but more ash from Icelands erupting volcano threatened to stall plans to reopen British airspace.

Although authorities in Iceland reported that eruptions from the Eyjafjallajokull volcano were weakening, volcanic ash already in the air continued to blow towards Britain.

The country's weather forecaster, the Met Office, said current wind patterns suggested no let up until the end of the week.

"At the moment current weather conditions aren't set to change until about Friday and then it looks like then the change is going to be south-westerly winds which will then take it away from the UK," a spokeswoman told CNN.

About 14,000 flights were expected to operate in European airspace Tuesday -- half of scheduled air traffic, according to Eurocontrol, the intergovernmental body that manages European air travel.

Flights were restricted to those above 20,000 feet, Eurocontrol said. Lower air space is closed or severely restricted across Europe, it added.

Planes were back in the air in France, Germany and Italy. Around 75 percent of scheduled flights were expected from France's two main airports, Charles de Gaulle and Orly, French Minister for Ecology Jean-Louis Borloo told French radio network RTL.

Germany's air authority, DFS allowed flights to take off and land in German airports at low altitude under visual flight rules to reduce the risk of damage to airline instruments. The country's main carrier Lufthansa said it hoped to operate as many as 330 flights, including some long-haul flights, Tuesday.

Italy has reopened its airspace nationwide, the Italian aviation authority ENAC announced.

While airports were open in Spain, the ban on flying in British airspace left thousands of Britons trapped in the country.

The British government dispatched a navy warship, the HMS Albion, to Santander to pick up 280 civilians and 450 British military personnel returning from Afghanistan, all of whom were stranded by the ash cloud.

The Albion is due to arrive at Portsmouth, England, late Wednesday, the British Ministry of Defence said.

The British government advised its citizens stranded within Europe to make their way to Calais, France, or any northern European port. It said it was working with Spanish authorities to establish an "air hub" in Madrid, where passengers arriving on flights from outside Europe could be transferred to northern French ports by train or bus.

Elsewhere in Europe, some countries opened their airspace to travel while keeping airports closed. Flights higher than around 20,000 feet were allowed through Danish airspace, the country's air traffic control service NAVIAIR said.

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Norway opened its part of its airspace near Bergen briefly on Tuesday and predicted Gardermoen, just north of Oslo, would be open all day, the country's air traffic control service AVINOR said.

However, Finland confirmed all Finnish airports would remain closed until 9 a.m. local time (2 a.m. ET) Wednesday.

Some Asian carriers including Singapore Airlines and Malaysia Airlines announced the gradual resumption of flights to Europe. And in India, authorities granted a two-week visa extension for Europe-bound foreign travelers stranded in the country because of the volcano.

While parts of Britain's airspace opened briefly in the country's north, a complete ban remained on flights to and from London's Heathrow and Stansted airports, the British air traffic authority NATS said.

British Airways issued a statement Tuesday apologizing to its customers for the continued disruption.

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The airline said it was forced to cancel plans to operate some short-haul and long-haul flights Tuesday, but said it hoped to run a number of inbound longhaul flights, subject to air restrictions.

It said it was working with other airlines to provide data to authorities which could lead to a reopening of British skies.

A British Airways test Sunday found no damage to the aircraft at a variety of altitudes, the airline said Monday.

"The analysis we have done so far, alongside that from other airlines' trial flights, provides fresh evidence that the current blanket restrictions on airspace are unnecessary," said Willie Walsh, British Airways' chief executive.

Finland's Ministry of Defense, however, said in a statement Monday that Finnish Air Force F-18 Hornet fighters that conducted test flights over northern Finland on Thursday showed effects from volcanic material.

"A post-landing inspection revealed accumulations of powdery volcanic dust in the aircraft's air intakes," the ministry said. Officials used a fiberscope to closely examine the engines of one aircraft, and "imagery ... indicated that even a short-duration flight inside an ash cloud may cause substantial damage to aircraft engines."

The engines will undergo a more thorough inspection, the ministry said.

Meteorologist Derrick Ryall of the MET Office in London said there is no "defined safe limit" for volcanic ash.

"No one has said what a safe limit for aircraft is, so therefore it's very difficult for these judgments to be made," he said.

Since the volcanic eruption worsened last week, sending an ash cloud into the skies, airlines have been losing at least $200 million a day, according to the International Air Transport Association, the trade group representing airlines. Airports had lost close to 136 million euros ($184 million) as of Sunday, said Olivier Jankovec, director general of Airports Council International, Europe.

More than 6.8 million passengers have been affected, Jankovec said in a statement, adding that the effect is worse than after the September 11 terrorist attacks on the United States.

Thousands of people have been living in airports.

"I have been living out of my carry-on for five days," said Paulo Wu, stranded in Amsterdam, Netherlands. After two nights sleeping on a cot at the airport, he was able to get into a hotel with an actual bed, he said. "I have no bags. My bags are somewhere, I think, at the airport. I just have my carry-on," he said.

The air disruptions have led to cancellations of numerous high-profile visits. Many world leaders, including President Obama, were unable to attend Sunday's funeral services for Polish President Lech Kaczynski.

U.S. Secretary of State Hillary Clinton canceled plans to travel to Helsinki, Finland, a State Department official said. Clinton was set to depart Washington on Tuesday. No decision has been made about whether she will keep plans to visit Tallinn, Estonia, the official said. If she does, she will not depart before Wednesday.

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