* 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 (Reuters) - A long-dormant volcano haserupted in Eritrea, monitors said on Monday, spewing a huge ashcloud across the Horn of Africa, threatening air travel andcurtailing a visit by U.S. Secretary of State Hillary Clinton.
The Dubbi volcano began belching plumes of ash at aboutmidnight on Sunday following a string of earthquakes in theremote, arid region close to the border with Ethiopia, whereClinton 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-basedVolcanic Ash Advisory Centre (VAAC) showed the eruption throwing an ashcloud 13.5 km (8.4 miles) up -- a potential blight on airlines.
"It hasn't affected our operations yet, but we are observingthe situation closely with experts at Addis Ababa Universityobservatory," Ethiopian Airlines spokesman Getachew Tesfa said.
Germany's Lufthansa (LHAG.DE) said on its website that ithad cancelled a flight out of the Eritrean capital Asmara onMonday and another flight into Addis Ababa. It gave no reasonfor the cancellations.
U.S. officials said they had been told Ethiopia wasconsidering shutting down Addis Ababa's main internationalairport as the ash cloud headed toward the capital. There was noimmediate comment from Ethiopia's Civil Aviation Authority.
Satellite images suggested Sudanese airspace could also beaffected.
Dubbi is located 350 km (219 miles) north of Asmara and 233km (146 miles) east of the Ethiopian city of Mekelle.
The independent earthquake monitoring websiteearthquake-report.com said it might be another nearby volcanonearby known as Nabro that was erupting and carried testimoniesfrom residents in the region confirming the ash cloud.(Additional reporting and writing by Barry Malone in Kampala;Editing by Richard Lough and Dan Williams)