Massive boulders crashed onto houses Wednesday in Mathare, a Nairobi slum, killing at least six people and trapping many underground after a night of heavy rains, police and the Kenyan Red Cross said.

"Two other bodies have further been retrieved now bringing the number of those who have lost their lives in the Mathare tragedy to six people," the Red Cross said in a statement.

"The main challenge at the moment is accessibility. The appropriate machinery for rescue cannot access the scene of the tragedy," it said.

Police chief Mathew Iteere had earlier put the death toll at four. Families at the scene told AFP several people were still missing.

The Red Cross said more than 40 houses were destroyed in the sprawling slums in the north of the capital when huge boulders came crashing onto them early Wednesday morning, the Kenya Red Cross said.

Teams of rescue workers used hoes and axes in an attempt to break up boulders that towered over their heads as crowds of onlookers stood perilously close.

"The rescue efforts have been hampered by heavy rains, lack of access and appropriate machinery and crowds of people who are slowing down the whole process," the Red Cross said.

Teams from the military joined in the rescue effort in early afternoon.

"It's by God's grace that we are alive with my family," said Rose Okoth, a mother of three who managed to run out of her house in time.

"I heard a loud thud. I thought it was a thunderstorm then I heard screams and that is when I ran out to safety with my children. A few minutes later another rock fell on our house. It could have killed us," she told AFP.

An AFP reporters said the cries from under the rubble that had been heard earlier in the day had subsided by early afternoon.

Heavy rains have begun pounding several Kenyan regions at the start of the main rainy season, which often causes floods and massive displacement.