"There is less activity... A lot less of the ash is going into the atmosphere," Petur Matthiasson, a spokesman for Iceland's Civil Protection and Emergency Management Administration, told AFP.

The plume of ash from Grimsvoetn, located in the southeast of Iceland at the heart of its largest glacier, Vatnajoekull, had dropped from its peak of 20 kilometres (12 miles) in the hours after the eruption began to between three and five kilometres Tuesday morning, according to official measurements.

"This is good news for aviation at least," Mattiasson said, pointing out while "there is still a lot of ash fall in the vicinity of the volcano in the south of Iceland," the lower column of ash meant "there is a lot less going into the atmosphere and going into the jetstream."

By Tuesday, hundreds of flights, especially to and from Scotland, were cancelled as the ash blew over Britain, but experts said they did not expect to see the same level of air travel disruption as last year, when a cloud of ash from the nearby Eyjafjoell volcano grounded more than 100,000 flights and left eight million passengers stranded.

Mattiasson explained that while "less ash is going into the upper atmosphere ... the ash that went into the air in the beginning is still floating away."

"It went north and is coming back with the jetstream, so that's the problem at this time," he said.

Olof Baldursdottir, a spokeswoman for the Icelandic Meteorological Office, said some of the decline in the height of the ash could probably be attributed to "strong winds that affect the plume."

Another explanation for the drop could be "that the ice is no longer melting into the crater," making the eruption less explosive, said Susan Stipp, a professor at the Nano-Science Centre at the University of Copenhagen who has headed a study published last month on the dangers posed by the Eyjafjoell ash cloud.

That, she explained, would mean "we're going to have more typical ash which is larger particles," she said, hinting the ash could become heavier and thus fall out quicker.

The eruption could thus change and enter "a more traditional phase of volcanic eruptions, the kind that you get in Indonesia, in Hawaii, in Italy and so on," Stipp said.

Baldursdottir meanwhile said it remained "very difficult to predict" when the eruption would end.

Grimsvoetn is Iceland's most active volcano, erupting nine times between 1922 and 2004, but its eruptions have usually been quite short.

Baldursdottir however pointed out that the same volcano had in 1873 erupted for seven months, but stressed that for most of that period it was a "low-intensity" eruption.

The ongoing eruption has meanwhile shown the most violent start ever registered at Grimsvoetn, and the peak of its plume towered at around double the height of the column spewed out last year by Eyjafjoell.