Eighteen people died and dozens were missing around the wine-growing hub of Ahrweiler, in Rhineland-Palatinate state, police said, after the Ahr river that flows into the Rhine rose and brought down half a dozen houses.

Eight people died in the Euskirchen region south of the city of Bonn, authorities said. In Belgium, two men died due to torrential rain and a 15-year-old girl was missing after being swept away by a swollen river.

Hundreds of soldiers were helping police with the rescue efforts, using tanks to clear roads of landslides and fallen trees, while helicopters winched those stranded on rooftops to safety.

The floods have caused Germany's worst mass loss of life in years. Flooding in 2002 killed 21 people in eastern Germany and more than 100 across the wider central European region.

Chancellor Angela Merkel expressed her dismay.

"I am shocked by the catastrophe that so many people in the flood areas have to endure. My sympathy goes out to the families of the dead and missing."

Armin Laschet, the conservative candidate to succeed Merkel as chancellor at a general election in September and the premier of the hard-hit state of North Rhine Westphalia, rushed to the flooded area and blamed the extreme weather on global warming during a visit to the area.

In Belgium, about 10 houses collapsed in Pepinster after the river Vesdre flooded the eastern town and residents were forced to leave more than 1000 homes.

Downstream in the Netherlands, flooding rivers damaged many houses in the southern province of Limburg, where several care homes were evacuated.

As well as the eight who died in the Euskirchen region, another seven people, including two firefighters, died elsewhere in North Rhine-Westphalia, several of them in flooded cellars.

"It's a catastrophe. There are dead, missing and many people still in danger. All of our emergency services are in action round the clock and risking their own lives," said Malu Dreyer, premier of the Rhineland-Palatinate.

Further down the Rhine river, the heaviest rainfall ever measured over 24 hours caused flooding in cities including Cologne and Hagen, while in Leverkusen 400 people had to be removed from a hospital.

Weather experts said rain in the region during the past 24 hours had been unprecedented, as a near-stationary low-pressure weather system also caused sustained local downpours to the west in France, Belgium and the Netherlands.

Rainwater draining into the Rhine was expected to test flood defences along the river, including in Cologne, on the lower Rhine, and Koblenz, where the Rhine and Moselle merge.

More heavy rain was due in southwestern Germany, on the upper reaches of the German Rhine, later on Thursday and Friday, the German Weather Service said.

Australian Associated Press