* Volcano spews ash cloud 13.5 km high - VAAC
* Forces Hillary Clinton to curtail Africa visit
* Volcano dormant since 1861
(Recasts with Clinton, details, previous KAMPALA)
By Aaron Maasho
ADDIS ABABA, June 13 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 U.S. 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. [nLDE75C184]
Dubbi is thought to have last erupted in 1861.
The U.S. Geographical Survey said the biggest quake had measured 5.7. Charts on the website of the France-based Volcanic Ash Advisory Centre (VAAC) showed the eruption throwing an ash cloud 13.5 km (8.4 miles) 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 spokesman Getachew Tesfa said.
Germany's Lufthansa (LHAG.DE) 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.
U.S. 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 350 km (219 miles) north of Asmara and 233 km (146 miles) east of the Ethiopian city of Mekelle.
The independent earthquake monitoring website earthquake-report.com 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. (Additional reporting and writing by Barry Malone in Kampala; Editing by Richard Lough and Dan Williams)