The volcanic ash spewed by Mount Sinabung in Karo regency, North Sumatra, disrupted a number of flights from Kuala Namu International Airport in Deli Serdang regency on Sunday.
In Bandung, the Center for Volcanology and Geological Hazard Mitigation (PVMBG) increased the volcano's status to 'awas' (beware), the highest level of the four-level alert system.
Affected airlines included Susi Air, which canceled five flights while Air Asia postponed its flight to Penang, Malaysia, citing safety reason as volcanic ash has reached the airport.
Susi Air operations manager Hadi Zulfadli said the flights included those from Kuala Namu to Simeulue and Singkil in Aceh and Aek Godang in Padang Sidempuan regency, North Sumatra. Also canceled was the flight from Banda Aceh to Kutacane.
Hadi said all the routes were unsafe because of the ash.
'The ash was quite thick. In the morning, all of our aircraft at Kuala Namu had been covered by ash,' he said on Sunday.
'It is dangerous if we keep flying so we decided to temporarily halt our operations.'
Hadi said flights would resume if the situation returned to a safe level.
Kuala Namu operations manager Ali Sofyan said that so far only Air Asia and Susi Air flights were affected by the volcanic ash.
'In general, the airport operations are normal. We found volcanic ash at a number of areas in the airport, such as the runway, but they have been cleared,' he said.
Mt. Sinabung erupted at about 9:45 p.m. Saturday, spewing volcanic ash as high as 10 kilometers.
The ash reached the provincial capital of Medan, some 80 kilometers away, for the first time since Sinabung erupted in 2010. This caused Medan residents to panic and become concerned about the ash, which started to fall at 11:30 p.m. The affected areas included Medan Tuntungan, Medan Johor, Medan Selayang and even as far as Belawan.
Meanwhile, Karo regency administration spokesman Jhonson Tarigan said evacuees had continued to arrive from various villages since the volcano erupted on Saturday.
'Currently there are 11,618 evacuees coming from 19 villages,' he said.
To anticipate a surge of evacuees, the regency had prepared 26 evacuation centers but only 22 are currently occupied.
North Sumatra Health Agency head Siti Hatati Surjantini called on residents in a number of regencies and cities around Mt. Sinabung to be vigilant. She asked residents to wear glasses and masks while outside and to reduce outdoor activities.
'I have sent a warning circular to the health agencies in Binjai, Dairi, Deli Serdang, Langkat, Medan, Pakpak Bharat and Serdang Bedagai to help monitor the eruption's impact,' she said.
She added that her office had sent 180,000 masks, medicine and four tons of baby food to replenish health posts in Karo regency.
PVMBG head Hendrasto said that the wind was blowing toward Medan when Mt. Sinabung erupted, bringing ash to Medan.
He admitted that there was a lot of volcanic activity, hence the alert status increase as of 10 a.m. on Sunday.
'The volcanic activity is still fluctuating. It is accumulating so the pressure is high with high amounts of gas,' Hendrasto said on his decision to increase the alert level.
'Another indication is that we found volcanic rocks of between 3 and 4 centimeters in diameter within a 3 to 4 kilometer radius from the Sinabung crater.'
Hendrasto also recommended implementing an exclusion zone around the crater with a 5 kilometer radius as well as evacuating residents from four additional villages outside the 5-kilometer radius because the four villages were located in the mouth of the Mt. Sinabung crater,' he said.
National Disaster Management Agency (BNPB) spokesman Sutopo Purwo Nugroho estimated that another 15,000 villagers would come to evacuation centers following the status increase.