Seven people from the Comoros, including a pregnant woman and her children, died of starvation when their boat broke down and  drifted over 1,100 kilometres to the Kenyan coast, a coastguard official said Wednesday.

Survivors told of their grim three-week ordeal when disaster struck shortly after 15 family members set off on a two-day voyage from the Comoros island of Ndzuwani to France's Indian Ocean territory of Mayotte.

The boat's propeller fell into the water and heavy winds pushed the vessel towards the Kenyan resort of Malindi, over 1,100 kilometres (700 miles) away, Kenya Coast Guard Service's head of public affairs Kenneth Sabila told AFP. 

"They were helpless and could not control the boat," he said, adding that they were at the mercy of the strong Indian Ocean waves for 22 days.

The pregnant woman, her husband and two children were among the seven who died when food rations ran out.

Surviving relatives told Kenya's Daily Nation they were forced to throw their decomposing bodies overboard to stay afloat.

The eight who made it to Kenya survived on raw fish and salt water, and were spotted by local fishermen on Monday, unconscious, weak and dehydrated. 

One of the survivors, Mahmoud Hassan, told the Daily Nation that all 15 on board belonged to one family.

"It's by God's grace that I'm alive," another, Nassiru Tawidu, 28, was quoted as saying, through tears.

One of the fishermen who found the survivors told the paper that the boat was in bad shape and they could not have survived another night on the high seas.

"The Kenyan navy sent medical personnel to attend to them," said the coastguard's Sabila, adding that the authorities had been in touch with their relatives through the Comoros consulate.

The family's reason for the voyage was not immediately known.

Mayotte has in recent years seen a massive influx of migrants from the Comoros, its comparatively poor and unstable neighbour, and increasingly also from countries in sub-Saharan Africa.