Authorities in Norway are considering blowing up part of a dam at risk of bursting.

Days of heavy rain have triggered landslides and flooding in mountainous southern Norway, where the measure is being considered to prevent downstream communities from getting deluged.

The Glama, Norway’s longest and most voluminous river, is dammed at the Braskereidfoss hydroelectric power plant, which was under water and out of operation. Police said a controlled explosion before the dam fails would allow officials to control the flow of water.

“When there is so much water, we can perhaps imagine, in the worst case, a kind of tidal wave coming sailing down the river,” police spokeswoman Merete Hjerto told Norwegian broadcaster NRK.

Pal Erik Teigen, a police officer in the Innland region, later told the VG newspaper that the facility “is without power, so we cannot make contact with the hatches. We will soon physically help open a hatch”.

A Norwegian woman in her 70s died early on Wednesday after falling into a stream the day before. She managed to crawl up onto the banks, but police said because of the floods, it took several hours before rescue teams could bring her to the hospital, where she died.

Police in southern Norway said more than 600 people were evacuated in a region north of Oslo overnight and said the situation there was “unclear and chaotic”. The Norwegian Public Roads Administration said all main roads between Oslo and Trondheim, Norway’s third-largest city, were closed.

”We are in a crisis situation of national dimensions,” Innlandet country mayor Aud Hove said. “People are isolated in several local communities, and the emergency services risk not being able to reach people who need help.”

Storm Hans has battered parts of Scandinavia and the Baltics for several days, causing rivers to overflow, damaging roads and injuring people with falling branches.

More heavy rain was expected over southern Norway and central Sweden on Wednesday, as sheds, small houses and mobile homes floated in rivers or were carried away by strong currents.

Norwegian meteorologists said that up to 1.2 inches of rain could be expected by Wednesday evening, saying: “The quantities are not extreme, but given the conditions in the area, the consequences may be.”

In Goteborg, Sweden’s second-largest city, large parts of the harbor were under water.

The meteorological institutes for both countries have issued extreme weather warnings.

“This is a very serious situation that can lead to extensive consequences and damage,” the Norwegian Meteorological Institute said. Its Swedish counterpart issued a red warning for the west coast, saying “very large amounts of rain causing extremely high flows in streams” could be expected.

Erik Hojgard-Olsen, a meteorologist with the Swedish Meteorological and Hydrological Institute, was quoted by the Aftonbladet newspaper as saying that the weather was unusual for this time of year.

“It is exceptional to have such a low pressure (system) as Hans, which has brought so much rain for several days in a row,” he said. “Especially for being a summer month, it has lasted a long time.”

The Norwegian Water Resources and Energy Directorate upgraded its warning for floods and landslides from orange to red for parts of southern Norway. The directorate said record high flood levels were recorded in several places in the Drammensvassdraget, a drainage basin west of Oslo, the capital.

Erik Holmqvist, a senior engineer at the agency, said four lakes, including the Randsfjorden, the fourth-largest in Norway, were particularly vulnerable to flooding.

“We have to go all the way back to 1910 to get the same forecasts for the Randsfjorden,” Mr Holmqvist told the VG newswpaper.

Norwegian Prime Minister Jonas Gahr Store visited the affected areas of southern Norway. He said: “When the rain stops, another challenge begins: the water needs to get out.”