GARISSA, Kenya (Reuters) - Kenyan security forces investigated reports on Thursday that up to 25 people were killed in fighting between cattle rustlers in the Rift Valley.

An official on the border between Baringo and Samburu districts said the clashes late on Wednesday pitted young men from the Samburu tribe against Pokot herders.

The fighting was not linked to violence in which at least 1,200 people were killed after Kenya's disputed election in December, but the post-ballot turmoil also showed up tensions over land and tribe.

"We were first told 18 people were killed and later somebody else reported 25 killed. The government has dispatched security men to verify," said the official, who asked not to be named.

Six bodies were brought to a local mortuary, he said, while six people wounded in the violence were treated in hospital.

Samburu police commander Charles Wasike declined to comment on casualties, but said an investigation was underway.

Fights over water, cattle and pasture have long plagued Kenya's remote corners, particularly during droughts. Officials said last month that hundreds of pastoralists further north were migrating over the border to Ethiopia in search of water.

(Reporting by Noor Ali; Writing by Daniel Wallis)