As the rainy season takes hold in Indonesia, cold lava now threatens people living on the slopes of a volcano on Bali island that has been continuously erupting and spewing smoke and ash since last week.

National Disaster Mitigation Agency spokesman Sutopo Purwo Nugroho has repeatedly warned locals in the vicinity to stay away from rivers, particularly those with headwaters on the slopes of Mt. Agung.

"Besides the increasing threat of Mt. Agung's eruption, heavy rains have increased possibilities of overflowing lahar (cold lava) in the areas around Mt. Agung," Sutopo posted on Twitter.

The rainy season began in October and will last until April, and precipitation is currently very high.

As activity at the 3,142-meter volcano increases, Sutopo said, pyroclastic materials spewed out will also increase, and rivers swollen by heavy rainfall will carry cold lava downstream to villages.

"Watch out for cold lava floods. Cold lava has been seen in some places on the slopes of Mt. Agung. Don't do any activities within the danger zone and around rivers. Go away from the edges of rivers when rain fails in the upstream of the rivers," Sutopo said.

Since the Center for Volcanology and Geological Hazard Mitigation raised the alert level of the volcano to the maximum 4 amid fears of a major eruption, no activities have been allowed within 8-10 kilometers of the crater.

Cold lava, which is also locally called "cold lahar," is a mixture of water and rock fragments spewed by a volcano that becomes cold because of the rains and flows down the slopes of the volcano, entering a river valley.

It looks like wet concrete and its speed can be as fast as 200 km per hour in steep areas, hitting whatever is in its way, such as bridges, houses or dams.

On Thursday, the cold lava was seen flowing very fast in Unda River in the village of Pasek Bali, about 28 km from the crater, turning the river's water black and muddy.

For cousins I Wayan Sutiasih, 39, and I Wayan Mudiasih, 37, however, it was a chance they should not miss, despite the danger warnings. They climbed to the dike walls of the river, taking selfies with the flowing cold lava as background, as did other local tourists.

The river and its dike have attracted local tourists to visit. It has also become a popular location for brides- and grooms-to-be to have their pre-wedding photographs taken, inspiring local authorities to regulate tickets for those taking such pictures.

"In the past (when the latest Mt. Agung eruption occurred in 1964), according to our parents, the lahar hit a wooden bridge above this river and damaged it. But now the bridge is made of iron, so I hope it'll be strong enough to support the lahar's attack," Sutiasih said.

Komang Sukada Putra, 19, a gardener in the dike compound, said the cold lahar arrived in the village at 8:15 a.m. Sunday, 15 minutes after he arrived for work, following heavy rain.

"Earlier, ash from Mt. Agung's eruption on Saturday covered our village as thick as 0.5 centimeter. It hurt my eyes, so I needed to wear glasses and a mask," Putra said.

Heavy rain on Sunday, however, swept the ash away from the village, while at the same time bringing cold lava down.

"The river's water suddenly turned grey and black and the cold lava killed all fish here. No fish is still alive inside the river now," he said.

The cold lava flow, according to him, is getting bigger in volume and darker from day to day.

"It is better if rains come for one day to clean up the ash and stop for another day to prevent more lahar to come," the gardener quipped.

Since July, Mt. Agung has been discharging small amounts of volcanic gases after being inactive since 1964. An eruption in 1963 killed 1,549 people.

The volcano spewed gray smoke and ash as high as 700 meters on Nov. 21, and again on Nov. 25 to twice that height, before beginning to emit lava on Nov. 26.

Thursday, ash from the volcano forced Lombok Praya international airport on Lombok Island, east of Bali, to close from 10:37 a.m. until midnight after wind is bringing the volcano's ash eastward.

As of Thursday, 43,358 residents whose houses are within the danger zones were sheltering at evacuation centers, according to the disaster agency. But 90,000 to 100,000 people may need to evacuate as the danger of Mt. Agung erupting more explosively is increasing.