The ash cloud from the Nabro volcano has caused several airlines to cancel their flights in the region [GALLO/GETTY]

A volcano in Eritrea has erupted for a third day but with reduced intensity, its ash cloud spreading out over Sudan, southern Egypt and towards Saudi Arabia and forcing the cancellation of some regional flights.

Emirates, the flagship carrier of Dubai, said it was cancelling its flight transiting through the Ethiopian capital, Addis Ababa, on Tuesday due to airspace restrictions around Ethiopia.

Ethiopian Airlines officials told the Reuters news agency they had cancelled flights to the Sudanese capital Khartoum, neighbouring Djibouti as well as several domestic flights to Ethiopia's north.

Kenya Airways said it had cancelled a flight from Addis Ababa to Djibouti but that otherwise all its flights were operating as usual.

The Nabro volcano began hurling plumes of ash at about midnight on Sunday after a string of earthquakes.

Scientists initially wrongly identified the source of the eruption in the region close to the Ethiopian border as the nearby Dubbi volcano.

"The ash's direction and its intensity were very high on Sunday, but this morning the Modis [monitoring] satellite shows a weakening," Atalay Arefe, natural sciences professor at Addis Ababa University, said.

Satellite images on the France-based Volcanic Ash Advisory Centre's (VAAC) website showed the cloud heading towards Saudi Arabia.

Not known to have previously erupted, Nabro burst into life after a string of earthquakes, the biggest of which measured 5.7, according to the US Geographical Survey. VAAC said the initial eruption threw an ash cloud 13.5km high.

The online Earthquake-Report also said the intensity of the eruptions appeared to be subsiding.

"Based on the data from VAAC, we can clearly see that the eruption is winding down. There is some remaining activity as ash clouds are still [being] blown in the atmosphere (up to) an altitude of 6,096 metres," Earthquake-Report said.

Authorities in Ethiopia and Eritrea reported no casualties around the volcano. It was hard to verify these reports because of the difficulty accessing the arid region.

Hilary Clinton, the US secretary of state, had cut short her stay in Africa by a day on Monday because the ash cloud risked leaving her stranded.