In this photograph made available by Nasa, a plume of smoke from an erupting volcano in southern Eritrea (top centre) is carried by winds blowing across northern Ethiopia. Airlines that travel through East Africa are keeping an eye on the ash cloud. Photo: AP, Nasa/GSFC and Modis Rapid Response

Addis Ababa - A long-dormant volcano has erupted in Eritrea, monitors said on Monday, spewing a huge ash cloud across the Horn of Africa, threatening air travel and curtailing a visit by American Secretary of State Hillary Clinton.

The Dubbi volcano began belching plumes of ash at about midnight on Sunday following a string of earthquakes in the remote, arid region close to the border with Ethiopia, where Clinton wrapped up regional talks to depart early.

Dubbi is thought to have last erupted in 1861.

The United States Geographical Survey said the biggest quake had measured 5.7 on the Richter scale. Charts on the website of the France-based Volcanic Ash Advisory Centre (VAAC) showed the eruption throwing an ash cloud almost 15km up - a potential blight on airlines.

“It hasn't affected our operations yet, but we are observing the situation closely with experts at Addis Ababa University observatory,” Ethiopian Airlines spokesperson Getachew Tesfa said.

Germany's Lufthansa said on its website that it had cancelled a flight out of the Eritrean capital Asmara on Monday and another flight into Addis Ababa. It gave no reason for the cancellations.

US officials said they had been told Ethiopia was considering shutting down Addis Ababa's main international airport as the ash cloud headed toward the capital. There was no immediate comment from Ethiopia's Civil Aviation Authority.

Satellite images suggested Sudanese airspace could also be affected.

Dubbi is located 350km north of Asmara and 230km east of the Ethiopian city of Mekelle.

The independent earthquake monitoring website said it might be another nearby volcano nearby known as Nabro that was erupting and carried testimonies from residents in the region confirming the ash cloud. - Reuters