Indonesian authorities are restricting tourists from visiting the country's most popular volcano because of concern a small eruption on Thursday could be a precursor to something much bigger.

Villagers, hikers and tourists were not allowed within 2.5 km (1.5 miles) of Mount Bromo, in the east of Java island, as the volcano spewed smoke hundreds of meters into the air, said disaster agency spokesman Sutopo Purwo Nugroho.

"Mount Bromo is still unstable. It is estimated that the eruptions will continue," Nugroho said.

Flights bound for nearby Malang airport were diverted as a precaution to East Java's capital, Surabaya.

Indonesia straddles the so-called Pacific Ring of Fire, a highly seismically active zone, where different plates on the earth's crust meet and create a large number of earthquakes and volcanoes.