NAIROBI - Kenyan soldiers will join police to fight "rampant incidents of banditry" across the arid northwest that have killed more than 100 civilians and 16 police officers over the past six months, officials said.

President William Ruto, who came to power in last August's election, has promised to restore security to the drought-stricken borderlands with Uganda, South Sudan and Ethiopia, where endemic cattle rustling has been aggravated by the proliferation of automatic weapons.

Interior Minister Kithure Kindiki said the situation in the northern Rift Valley Region constituted a national emergency and gave people in the area three days to surrender their firearms before being subjected to the "full force of the law".

"Painful and decisive measures must be taken effective immediately," Kindiki said in a statement late on Monday.

As part of the response, the army will be sent to six counties in the region on Wednesday, Defence Minister Aden Bare Duale said in a separate statement.

Kenya has long contended in the east with security threats from Somalia's al Shabaab, who make cross-border raids aimed at pressuring the country into withdrawing its troops from the African Union-mandated peacekeeping force in Somalia.

(Reporting by Aaron Ross and Hereward Holland; Writing by Hereward Holland; Editing by Nick Macfie)