Ireland's Ryanair said it would protest against "unnecessary" restrictions as airlines braced for the heavy cost of grounding planes.

Worries about the effect of the ash cloud pushed forward US President Barack Obama's planned departure from Ireland and he arrived on Monday night in Britain to begin a state visit.

Last year, ash from another Icelandic volcano caused 100,000 flights to be canceled, stranding 10 million passengers and costing the industry an estimated US$1.7 billion in lost revenue.

Iceland's President Olafur Grimsson said the latest eruption was on a "monumental" scale but would not wreak the same havoc on aviation.

Norway's airport operator said the ash cloud would cause some flight restrictions on its west coast and said the ash was expected to spread further across southern Norway later on Tuesday.