International airlines have cancelled flights to airports near Mount Merapi, as Indonesia's most volatile volcano continues to pump out clouds of ash.
Some flights from Singapore and Malaysia were cancelled, and the Indonesian government warned other airlines to avoid some routes because of the ash.
At least 38 people have been killed since Merapi began erupting last week.
Scientists have warned that the volcano could continue rumbling for some time.
"It could go on for weeks, even months," vulcanologist Subandrio told AFP news agency.
Early on Tuesday, boiling red lava was clearly visible in the crater of the volcano, which lies about 20 miles (30km) from the city of Yogyakarta.
Several times after dawn, huge clouds of ash and debris shot out from Merapi.
The slopes of the mountain and nearby areas are covered with grey ash, while almost 70,000 residents remain in government shelters.
AirAsia, which is based in Malaysia, cancelled four flights from Kuala Lumpur to Yogyakarta and nearby Solo, while Silk Air cancelled two flights between Singapore and Solo.
Both airlines said they were watching the situation carefully.
Transport ministry spokesman Bambang Ervan said airlines had been told to "use an alternative route for safety reasons due to the volcanic ash", AFP news agency reported.
Some domestic flights have already had to be rerouted to other airports, the Jakarta Post reported.