The death toll from flooding in eastern Belgium hit 27 on Saturday, as police went door-to-door seeking news of at-risk residents and the prime minister toured devastated towns.

In an update, the Belgian crisis centre said that, in addition to the 27 confirmed dead, 103 people are "presumed missing and out of contact."

But it stressed that these people were not necessarily dead or in immediate danger, but may have lost their cellphones or be unable to recharge their batteries in areas without power.

"We hope it's simply that we can't get in touch with them," a spokesman said.

Prime Minister Alexander de Croo toured the town of Rochefort and elsewhere in the basin of the river Meuse to see the aftermath of what he has branded an "unprecedented" flood.

"Terrible damage, and suffering too, but also enormous solidarity," he tweeted after touring Rochefort and Pepinster, in the heart of the crisis.

De Croo was joined by European Commission president Ursula von der Leyen in the river valleys of eastern Belgium near the border with her native Germany, also badly hit.

"My heart ached as I met people who have lost their homes, the savings of a lifetime," Von der Leyen said. 

"I told them: Europe is with you. We are with you in mourning and we will be with you in rebuilding."

Flood waters descended on densely inhabited valleys in the region around the Meuse river on Thursday, after days of intense rain.

By Saturday, the skies were clearing and the downpour had abated, but the retreating waters left scenes of devastation across 120 local government areas.

Police were going door to door, checking on residents, and De Croo has declared Tuesday, the eve of Belgium's national day, a day of official mourning.