Flights to East Africa have been severely disrupted as ash from an Eritrean volcano spreads across the region and heads towards Saudi Arabia.
Several airlines said they had stopped flying to Eritrea, Sudan, Djibouti and Ethiopia.
The ash cloud was now changing direction, from north-west to south-west and had reached Sudan and Egypt, an Ethiopian academic said.
The cloud followed a volcanic eruption in Eritrea at the weekend.
It forced US Secretary of State Hilary Clinton to cut short a three-nation tour of Africa on Monday.
Initially only the German airline Lufthansa cancelled flights to Eritrea and neighbouring Ethiopia.
Now Ethiopian Airways has said it has cancelled flights to Sudan and Djibouti.
Kenya Airways said it was no longer flying on the Ethiopia-Djibouti route and Dubai's Emirates airline said it had cancelled flights to the Ethiopian capital, Addis Ababa.
Satellite images obtained by the France-based Volcanic Ash Advisory Centre showed the cloud was moving towards Saudi Arabia, Reuters news agency reports.
But Atalay Ayele of the Geophysical Observatory Centre of Ethiopia's Addis Ababa University said the size of the ash cloud was decreasing.
"The ash's direction and its intensity were very high on Sunday, but... the Modis [Moderate Resolution Imaging Spectroradiometer] satellite shows a weakening," he said.
According to the US space agency Nasa, the satellite images confirm the eruption was at the Nabro volcano, which is not known to have erupted before.
Earlier reports had said it was the Dubbi volcano that had sent the plumes of ash 13km (eight miles) into the air.
Peggy Hellweg, a seismologist at the University of California, told the BBC the volcano could go on rumbling and spewing ash for some time.