Officials are on standby to divert flights away from Bali should the situation arise.
Flights will be diverted to 10 other airports in Indonesia, including Jakarta, Makassar, Surabaya, Balikpapan, Solo, Ambon, Manado, Praya, Kupang and Banyuwangi.
Transport minister Budi Karya Sumadi said: “The planes will be diverted to their nearest location or where it originally took off from.”
The volcano is being closely monitored by airlines, with 100 buses prepared to evacuate tourists.
Virgin Australia and Jetstar are taking extra fuel on flights from Australia, in case they need to turn around.
Singapore Airlines is offering customers the option to rebook or have their flights refunded, if they are travelling between September 23 and October 2.
Indonesian officials insist the island is still safe for tourists, but locals in the area say tourism numbers have dropped.
One tourist who is staying in Amed, on the east coast of Bali, said he feels “pretty safe”.
German national Christoph Lange told AFP: “We’ve got like 20, 30, 40 shakes where you could feel the earth shaking.
“While we were diving we had quite a few. Under water you can hear it, it’s pretty loud actually.”
Mount Agung has been shaking since August, with the alert level raised to its highest on Friday last week.
Indonesia lies on the Pacific “Ring of Fire”, straddling tectonic plates.
The volcano last erupted in 1963. Over 1,000 people were killed in the natural disaster.
