Mount Barujari, that forms part of Rinjani, was seen emitting a 2,000-metre high column of smoke Tuesday afternoon, prompting authorities to cordon off and evacuate the area within a radius of 3 km around the crater, Efe news agency reported.
Rinjani, the second highest volcano in Indonesia standing 3,726-metres high, attracts thousands of tourists annually for a three-day trek to the mountain top, which has a lake in its boiler.
A disaster mitigation agency spokesman, Sutopo Purwo Nugroho, said nearly 400 foreign and local tourists were registered since Sunday to climb the mountain.
"We are still searching for about 389 other tourists, mostly foreign tourists, to find out their condition and to evacuate them immediately," the Guardian quoted Nugroho as saying.
There have been no reports of injuries.
Indonesian authorities in October 2015 issued a volcanic alert after an eruption forced the evacuation of more than 1,000 people, including tourists and residents.
Last year, Mt. Rinjani erupted several times emitting columns of smoke and ash forcing the Lombok airport to shut down and suspend flights in the neighbouring island of Bali.
Tuesday's eruption delayed many flights between Australia and Bali for several hours, although flight schedules are back to normal now.
Indonesia sits on the Pacific Ring of Fire, an area of high seismic and volcanic activity which is home to more than 400 volcanoes, including at least 129 that are still active and 65 qualified as dangerous.
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