Kenyan authorities worked to clear fuel and recover damaged wagons on Monday a day after a train carrying 250 000 litres of petrol derailed at Mombasa and dumped part of its cargo into the Indian Ocean and onto a major highway, officials said.

Sunday's spill left the port city, Kenya's second largest city, cut off from the mainland for most of Sunday while police cordoned off the area to mitigate the risk of fire.

Thousands of road, plane and train travellers were stranded until the highway was reopened in the evening.

It was unclear how much of the petrol the train was carrying spilled into the ocean, police said, adding they had begun an investigation into what caused the accident.

On Monday, a team of engineers and other local experts were at work recovering five train wagons that were submerged in the ocean, Mombasa police chief Johnstone Ipara told Reuters.

The water that the wagons had careered into was slick with a substance that appeared to be a petrol product, a Reuters witness said.

The train, which had departed from Mombasa on Sunday, was travelling on a decades-old line that carries goods from ships that arrive at Mombasa's port to the capital Nairobi.

A new cargo line opened in January, following the inauguration of a passenger line linking the two cities last year. Kenya's government borrowed from China and hired a Chinese state-owned company to build the lines.