LONDON - A dense ash cloud from an Icelandic volcano blew toward Scotland, causing airlines to cancel Tuesday flights, forcing US President Barack Obama to shorten a visit to Ireland, and raising fears of a repeat of last year's huge travel disruptions in Europe that stranded millions of passengers.

Britain's Civil Aviation Authority said it appears that ash from the Grimsvotn (GREEMSH-votn) volcano could reach Scottish airspace early Tuesday and affect other parts of the UK and Ireland later in the week.

British Airways suspended all its flights for Tuesday morning between London and Scotland, while Dutch carrier KLM and Easyjet canceled flights to and from Scotland and northern England at the same time. Two domestic airlines also announced flight disruptions.

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Still, authorities say they don't expect the kind of massive grounding of flights that followed last year's eruption of the Eyjafjallajokull volcano in Iceland because systems and procedures have been improved since then and the cloud is currently not expected to move over continental Europe.

Pilots unions, however, expressed concerns that the ash could still be dangerous.

Obama, who had been scheduled to spend Monday night in Ireland, was forced to fly to London early because of the ash cloud - he landed at the capital's Stansted Airport late Monday. Last year's Icelandic eruption also forced a change in his schedule then, causing him to cancel a trip to Poland.

Glasgow-based regional airline Loganair canceled 36 Scottish flights scheduled for Tuesday morning, as well as some flights to Birmingham and Belfast. It said its flights between Scottish islands would be unaffected. Another small airline, Eastern Airways, which is based in northern England, also canceled all flights to and from Scotland on Tuesday.

"Our No 1 priority is to ensure the safety of people both onboard aircraft and on the ground," said Andrew Haines, chief executive of the CAA. "We can't rule out disruption, but the new arrangements that have been put in place since last year's ash cloud mean the aviation sector is better prepared and will help to reduce any disruption in the event that volcanic ash affects UK airspace."

Many airlines said authorities last year overestimated the danger to planes and overreacted by closing airspace for five days amid fears that the abrasive ash could cause engines to stall.

CAA spokesman Jonathan Nicholson said authorities this time would give airlines information about the location and density of ash clouds. Any airline that wanted to fly would have to present a safety report to aviation authorities in order to be allowed to fly.

He said most British airlines had permission to fly through medium-density ash clouds, but none had asked for permission to fly through high-density clouds, classified as having over 4,000 micrograms of ash per cubic meter.

Even at that concentration of volcanic ash, experts said the air would not look much different from airspace unaffected by the ash, but officials say the tiny particles in the ash can sandblast windows and stop jet engines.

The international pilots' federation warned that it believed the cloud still presented a potential danger to commercial aircraft despite developments since last year.

"It remains our view that when there is an unknown then it is always better to err on the side of caution," said Gideon Ewers, spokesman for the International Federation of Air Line Pilots' Associations.

Thurai Rahulan, a senior lecturer in aeronautics at Salford University in northwest England, said the technology on how to measure and monitor ash has improved, but aircrafts' ability to cope with ash has not changed.

"Aircraft manufacturers have made more resources available to conduct studies on tolerating higher concentrations of ash, but as far as I know, no possible improvements have yet made it to front line operations yet," he said.

The disruption in Scotland is being caused by the smaller of two ash clouds from the volcano. The main cloud was causing minor disruptions around Scandinavia.

Iceland's main airport, Keflavik, and domestic airport Reykjavik both reopened Monday after being closed for almost 36 hours. Grimsvotn began erupting Saturday.

Hjordis Gudmundsdottir, spokeswoman for the airport administrator Isavia, said the first flight to take off would be an Icelandair flight to London Heathrow.

"The outlook is good for Keflavik and other Icelandic airports in the coming 24 hours," said Gudmundsdottir. "We don't have a forecast for after that so we wait and see."

The Met Office, Britain's weather forecasters, said there have been no major changes in the forecast - that some ash will drift across UK airspace, mostly in Scotland and Northern Ireland, by Tuesday morning.

But the weather in the UK has been very unsettled in the past two days and will continue to be that way in the days ahead, making predictions difficult.

"When it's all over the place, it's a bit trickier to predict where things may go," said forecaster Charlie Powell.

An Icelandic meteorological official said the eruption already appeared to be getting smaller, but Thierry Mariani, France's transport minister, said it was too early to tell whether air travel over Europe would be affected by the eruption.

Mariani told Europe 1 radio that the composition of the cloud will be examined in the coming days and if the ash is found to be harmful to airplanes, countries may take a joint decision to close part of Europe's airspace.

"The priority must always remain to ensure security," he said.

UK Transport Secretary Phillip Hammond told the BBC that Britain had equipment in Iceland analyzing the ash as it comes out of the volcano, and equipment in the UK that analyzes the density of the ash.

"We won't see a blanket closing of airspace," he said.