Six police officers have been killed by herders in volatile Laikipia County, where at least 40 people have died this year due to clashes over land, a Kenya police report said Wednesday.

The herders from the Pokot Community opened fire at the Laikipia Nature Conservancy and killed the officers, the report seen by the Associated Press said.

The officers were inside a pair of vehicles and part of a unit that works to prevent livestock thefts. The report says four other officers were wounded in the attack.

"The second motor vehicle retreated as they engaged the raiders, who fled back to the ranch after robbing the (slain) officers of five G3 rifles, one AK rifle and one Jericho pistol and unknown number of ammunitions," the document said.

Hundreds of semi-nomadic herders invaded private ranches late last year to save their cattle from a prolonged drought which affected half the country. Rain has fallen since then, but the herders have refused to leave the ranches.

Kenya's military and police have been working to disarm and to drive the hundreds of herders and their animals off the properties, but their actions appear to have escalated the violence. Nearly 10 police officers have been killed in the conflict. The herders also fatally shot dead a former British soldier and wounded the Italian-born author and conservationist Kuki Gallmann.

When the military and police drive herders from one ranch they move into another, the Laikipia Farmers Association has said.

Security agencies shot and killed hundreds of animals to drive away the trespassers, herders who insisted on anonymity for fear of reprisals have said. President Uhuru Kenyatta has ordered the government to compensate the herders for animals killed by the security agencies, Mwangi Kiunjuri, cabinet secretary of the Ministry of Devolution and National Planning, said earlier this month.

The farmers' association has accused politicians campaigning ahead of a national election next month of inciting the herders to invade the ranches by telling them the owners' leases have expired and that herders can take over the land and distribute it among themselves.

Human Rights Watch said in a report Tuesday that the ongoing violence in the Rift Valley is causing insecurity and forced displacement that could potentially keep thousands of people from voting in the August election.

"The Electoral Commission should ensure that polling places remain accessible and operate in safe areas so voting can proceed," the report said.