OSLO: A dam on a Norwegian river partly collapsed after heavy rain on Wednesday, sending water gushing through a wide gap in the structure, and the prime minister said the country should be prepared for more floods in the coming days.

The Braskereidfoss power dam on the Glomma River, Norway's biggest waterway, was unable to open its hatches after its control room flooded.

Norway has evacuated more than 3,000 people this week as rivers swelled to their highest levels in at least 50 years, and homes and businesses have been submerged or swept away by landslides.

Main roads were closed and train services suspended across wide parts of southern Norway as rivers breached their banks, and authorities warned of more flooding to come in the days ahead as the water moves towards lower-lying coastal regions.

"The next two to three days will be absolutely critical and we'll be mobilising around the clock," Norwegian Prime Minister Jonas Gahr Stoere told a press conference.

RIVER FLOWS NORMALISE

Water levels at the Braskereidfoss dam later subsided, flowing in a controlled manner and avoiding major downstream flooding, a police spokesperson said.

Police earlier considered blowing one of the 45-year-old dam's hatches to help relieve the water pressure, but decided not to as the explosion would have been difficult to control.

A railway bridge collapsed on the line connecting Oslo to Trondheim, Norway's third-largest city, but there was no danger as trains had been suspended since Monday.

Innlandet county, one of Norway's worst-hit areas, said many people were isolated by the floods and that first responders may not be able to reach those in need.

"We're in an emergency situation of national dimensions," Innlandet Mayor Aud Hove said in a statement.

The government on Wednesday mobilised more helicopters to take part in evacuations, the ministry of justice and public security said.

Strong winds, intense rain and landslides hit different parts of the Nordic region in recent days, knocking out power lines in Finland, flooding villages in Norway and Sweden and bringing public transport to a standstill in hard-hit areas.

On Monday, a Swedish train derailed when a railway embankment was washed away by floods, injuring three people.

Authorities in Norway and Sweden maintained red alerts, their most severe flood warnings, for several regions on Wednesday