Emergency workers in western Germany and Belgium rushed on Friday to rescue hundreds of people in danger or still unaccounted for as the death toll from devastating floods rose to more than 125 people.

Authorities in the German state of Rhineland-Palatinate said 63 people had died there, including 12 residents of an assisted-living facility for disabled people in the town of Sinzig who were surprised by a sudden rush of water from the nearby Ahr River.

In neighbouring North Rhine-Westphalia, state officials put the death toll at 43, but warned that the figure could increase.

German President Frank-Walter Steinmeier said he was “stunned” by the devastation caused by the flooding and pledged support to the families of those killed and to cities and towns facing significant damage.

“In the hour of need, our country stands together,” Steinmeier said in a statement. "It's important that we show solidarity for those from whom the flood has taken everything."

A harrowing rescue effort unfolded in the German town of Erftstadt, southwest of Cologne, where people were trapped when the ground gave way and their homes collapsed.

"We managed to get 50 people out of their houses last night," county administrator Frank Rock told German broadcaster n-tv.

Aerial photos showed what appeared to be a massive landslide at a gravel pit on the town's edge.

"One has to assume that under the circumstances some people didn't manage to escape," Rock said.

Authorities were trying to account for hundreds of people listed as missing, but they cautioned that the high number could be due to duplicated reports and difficulties reaching people because of disrupted roads and phone service.

After Germany, where the death toll stood at 106, Belgium was the hardest-hit by the floods that caused homes to be ripped away.

Belgian Interior Minister Annelies Verlinden told the VRT network on Friday that the country had confirmed the deaths of 20 people, with another 20 still missing.

Water levels on the Meuse River that runs from Belgium into the Netherlands remain critical, and several dikes were at risk of collapsing, Verlinden said. Authorities in the southern Dutch town of Venlo evacuated 200 hospital patients due to the looming threat of flooding from the river.

Flash floods this week followed days of heavy rainfall in Western Europe. Thousands of people remained homeless in Germany after their houses were destroyed or deemed at-risk by authorities.

The governor of North Rhine-Westphalia, who is hoping to succeed Chancellor Angela Merkel as the nation's leader after Germany's election on Sept 26, said the disaster had caused immense economic damage to the country's most densely populated state.

"The floods have literally pulled the ground from beneath many people's feet," Governor Armin Laschet said at a news conference. "They lost their houses, farms or businesses."

Federal and state officials have pledged financial aid to the affected areas.

Malu Dreyer, the governor of Rhineland-Palatinate state, said the disaster showed the need to speed up efforts to curb global warming. She accused Laschet and Merkel's centre-right Union bloc of hindering efforts to achieve greater greenhouse gas reductions in Germany — Europe's biggest economy and a major emitter of planet-warming gases.

"Climate change isn’t abstract anymore. We are experiencing it up close and painfully," she told the Funke media group.

Steinmeier, the German president, echoed her calls for greater efforts to combat global warming.

"Only if we decisively take up the fight against climate change will we be able to limit the extreme weather conditions we are now experiencing," he said.

Experts say such disasters could become more common in the future.

"Some parts of Western Europe ... received up to two months of rainfall in the space of two days. What made it worse is that the soils were already saturated by previous rainfall," World Meteorological Organisation spokesperson Clare Nullis said.

She added: “Climate change is already increasing the frequency of extreme events. And many single events have been shown to be made worse by global warming.”

Defence Ministry spokesman Arne Collatz said the German military had deployed over 850 troops to help with flood efforts but the number was “rising significantly because the need is growing”. He said the ministry had triggered a “military disaster alarm”.

Italy sent civil protection officials, firefighters and rescue dinghies to Belgium to help in the search for missing people from the devastating floods.

In the southern Dutch province of Limburg, which also has been hit hard by flooding, troops piled sandbags to strengthen a 1.1-kilometre (0.7 mile) stretch of dike along the Maas River and police helped evacuate low-lying neighbourhoods.

Caretaker Dutch Prime Minister Mark Rutte said the government was officially declaring flood-hit regions a disaster area, meaning businesses and residents are eligible for compensation.

Dutch King Willem-Alexander visited the region on Thursday night and called the scenes "heart-breaking".

Meanwhile, sustained rainfall in Switzerland has caused several rivers and lakes to burst their banks.

Public broadcaster SRF reported that a flash flood swept away cars, flooded basements and destroyed small bridges in the northern villages of Schleitheim and Beggingen late on Thursday.

Erik Schulz, the mayor of the hard-hit German city of Hagen, 50km (31 miles) northeast of Cologne, said there had been a wave of solidarity from other regions and ordinary citizens to help those affected by the floods.

“We have many, many citizens saying ‘I can offer a place to stay, where can I go to help, where can I register, where can I bring my shovel and bucket?’,” he told n-tv. “The city is standing together and you can feel that.”