In the worst single incident for Kenyan police in living memory, 38 officers were killed in northern Kenya over the weekend by heavily armed cattle rustlers, according to injured police reservists who were on the scene, CNN reports.

Local media reports put the number at between 37 and 42 dead. Internal Security Minister Katoo Ole Metito would not give a total figure on how many police officers had died.

The police officers were part of an operation to recover cattle belonging to the Samburu tribe that had been stolen by the Turkana tribe, according to the minister. They were ambushed in a valley when a large group of Turkana fired on police, leaving dozens dead and many injured.

The Turkana and Samburu, both pastoralist groups, have frequently engaged in tit-for-tat cattle rustling operations over many years. Villages in the region are heavily fortified by residents, and it's not unusual to see cattle herders armed with automatic rifles.

In the most recent incident, Turkana raided the Samburu last month and took a large amount of cattle, according to the internal security ministry.

The Kenyan government intervened and gave an ultimatum for the Turkana to return the cattle last week, or it would respond with force. The Turkana did not meet the deadline, prompting the police operation that ended in tragedy.

Both Turkana and Samburu leaders have frequently asked for more security in the region from the government. They say the police are outmanned and outgunned.