The eruption is believed to have followed a string of earthquakes in the region.
The last know eruption of this volcano known as Dubbi was in 1861.
Peggy Hellweg, a seismologist at the University of California, gave the BBC's Network Africa programme some more information on the activity. Source
Despite having undergone no historically reported eruptions,the Nabro Volcano likely erupted shortly after midnight on June 13, 2011 local time, after a series of earthquakes in the Eritrea-Ethiopia border region, ranging up to magnitude 5.7. The ash plume was observed on satellite drifting to the west-northwest along the border, spanning about 50 km wide and several hundred kilometres across in the hours immediately following the reported eruption, while reportedly reaching 8 miles (15km) high.
On June 13 US Secretary of State Hillary Clinton had to cut short a trip to Ethiopia due to the ash cloud, which was projected to enter a west-to-east jetstream and enter the atmosphere of countries such as Egypt, Yemen, Israel, Iraq, Jordan, Sudan, Somalia, Djibouti and Saudi Arabia.
The ash cloud began disrupting air traffic as UAE-based Emirates flights were cancelled along with Saudi Arabian Airlines flights. Luxor International Airport in Luxor, Egypt was placed on a state of emergency. Source