Gunung (Mount) Sinabung is an active volcano located around 50km (by air, much further by road) from Bukit Lawang, in the Karo region of North Sumatra. The closest towns are Berastagi and Kabanjahe, around 14km distant. It is around 55km from Medan, and further still from Medan's Kuala Namu airport.
The volcano was dormant between 1600 and 2010. Since late 2013 it has been erupting every few months. As of June 2015 the volcano is actively erupting. The flow of lava and gas from the volcano has reached a distance of up to around 3km at the furthest from the volcano. The flow from the volcano has been primarily to the southeast. Areas of the north side of the volcano generally have not been affected by volcanic flows, however there are various vents on the north side of the volcano that could open up.
Eruptions from the volcano have resulted in the abandoning of several small villages close to the mountain. These villages on the slopes of the mountain benefited from the fertile volcanic soil. Frequent eruptions have resulted in a semi-permanent evacuation of these villages, as a previous return to the villages was followed by a further eruption killing more than a dozen people. In total around 20,000 people have been displaced by the volcano.
For tourists visiting Bukit Lawang and North Sumatra, please understand that Sinabung is a long way from Medan, Bukit Lawang and Lake Toba, so these areas are not threatened or at risk from volcanic eruptions. During eruptions, Berastagi itself is likely to be affected by the ash cloud, affecting breathing for those in the area. Occasionally the ash can be blown by the wind as far as Medan.
As a result of the eruptions, it is not possible to climb Sinabung volcano at any time. The other tourist attraction affected is Lau Kawar lake, which is situated northwest of the volcano, and during eruptions it is likely to be out-of-bounds though at other times you can still visit, although only to see the lake, as its previous base as a starting point to climb the volcano is now redundant with no climbing going on.