A dam has burst on a flower farm in Kenya's Rift Valley after weeks of torrential rain, unleashing a sea of water that smashed into two villages and killed at least 47 people.

Key points:

• Floodwaters swept away powerlines, homes and buildings, including a primary school
• About 40 people have been rescued from the mud and taken to hospitals
• 450 homes were hit by floodwater

The walls of the Patel dam, situated on a large commercial farm on top of a hill in Nakuru county 190 kilometres northwest of Nairobi, gave way late on Wednesday as nearby residents were sitting down to their evening meals.

"Many people are missing. It is a disaster," said Rongai town police chief Joseph Kioko.

The floodwaters swept away powerlines, homes and buildings, including a primary school, a photographer on the scene said, as rescue workers picked through rubble and mud searching for survivors.

The bodies of two female victims were found several kilometres away.

A rescue operation by Kenya Red Cross and Nakuru County disaster management teams went into operation and up to 40 people have been rescued from the mud and taken to hospitals.

Many are feared trapped under the mud.

"The water has caused huge destruction of both life and property," Nakuru's governor Lee Kinyajui said in a statement.

"The extent of the damage has yet to be ascertained."

In Solai, 67-year-old Veronica Wanjiku Ngigi said she was at home brewing tea with her son at about 8:00pm (local time) when his wife rushed in to say the dam had burst and they needed to get to higher ground immediately.

"It was a sea of water. My neighbour was killed when the water smashed through the wall of his house," she said.

"He was blind so he could not run. They found his body in the morning. My other neighbours also died. All our houses have been ruined."

Local resident Desmond Gitonyi said an entire village was swept away.

Mr Gitonyi, who lives close to the village, posted photos the following morning of families searching through pools of mud.

Mr Gitonyi said there was panic at the scene and at the Nakuru Hospital as people searched for their missing loved ones.

Concrete foundations were the only thing left behind as entire houses had been swept away.

"The Interior Cabinet Secretary Fred Matiang'i has just arrived at the scene," Mr Gitonyi said.

"The rescue mission is still underway."

Hundreds of homes hit by floodwater

Families searched the ruins of their homes the morning after the dam broke.

After a severe drought last year, East Africa has been hit by two months of heavy rain affecting nearly a million people in Kenya, Somalia, Ethiopia and Uganda where bridges have been swept away and roads turned into rivers of mud.

Nakuru lies in the heart of Kenya's fertile Rift Valley, home to thousands of commercial farms.

The region is dotted with irrigation reservoirs built in the last two decades to meet the demands of the rapidly expanding agricultural sector.

Nakuru Governor Lee Kinyanjui said 450 homes had been hit by the floodwaters and safety engineers had been sent to inspect three other dams nearby.

Ms Wanjiku said at least one more looked like it too was ready to burst.

"There is another dam which is also overflowing which is looking risky," she said. "We are scared.

Even before this week's dam-burst, heavy rains had caused havoc in Kenya, killing 132 people and displacing 222,000, according to the Government.

Roads and bridges have been destroyed, causing millions of dollars of damage.

The United Nations UNOCHA disaster agency said 580,000 people had been affected by torrential rain and flooding in neighbouring Somalia, while the Somali region of eastern Ethiopia had taken a hammering, with 160,000 people affected.

UNOCHA said the flooding could get worse, with heavy rains forecast to continue in the Rift Valley and the Lake Victoria basin over the next few weeks.