Several airlines have temporarily suspended flights between Singapore and Jakarta because of volcanic ash spewing from Mount Merapi in central Java.
Among those affected were Singapore Airlines, Japan Airlines, Malaysia Airlines, Lufthansa and Cathay Pacific.
Merapi, one of the world's most active volcanoes, began erupting two weeks ago and has killed more than 130 people.
Airline officials say the ash is a safety threat and could damage aircraft.
Domestic flights have been unaffected.
Azharuddin Osman, director of operations for Malaysia Airlines, said "the volcanic ash presence in the airways surrounding Jakarta could cause severe damage to our aircraft and engines, which could impair the safety of our operations."
Indonesia has already been forced to close its international airport at Yogyakarta and divert flights to the nearby city of Solo. A Silkair flight to Solo was also cancelled.
Mount Merapi has been producing thick clouds of ash and superheated gases in its biggest eruption in a century. In addition to the loss of life, some 200,000 people have been evacuated from the area.
Officials say about 85 people died when the last major eruption on Friday blasted hot ash and molten debris into local communities. Scores more suffered severe burns.
A mass burial is being prepared for dozens of victims from the village of Argomulyo 18km (11 miles) from the crater.
Rescue teams were still picking through the rubble of villagers' homes on Saturday.
One rescuer, named Utha, told AFP news agency: "I found three bodies - a child, mother and father, still in their bed. They must have been sleeping when the hot ash struck their house.
"We also found a dead man with a phone still in his hand."