Vice President Ruto and his family were not home at the time of the attack, which multiple sources previously said was carried out by armed gunmen. The house is guarded by the General Service Unit (GSU), an elite police force.

"In circumstances that are yet unclear, he (the attacker) hit an officer on duty ... with a machete and managed to enter a farm complex," Inspector General of Police Joseph Boinnet said in a statement.

Ruto has an "expansive" home near the town of Eldoret, about 325 kilometers (200 miles) northwest of the capital, Nairobi.

"More security personnel have been deployed and a security operation is ongoing," an unnamed officer said earlier.

"For now we have nothing to tell you until the operation is over. Just give us time," Wanyama Musiambo, Rift Valley Regional Coordinator, told reporters at the scene.

Ruto's running mate, President Uhuru Kenyatta, is in a tough re-election battle, slated for August 8, against longtime opposition leader Raila Odinga. Kenyatta is seeking a second and final term in office.

 Rift Valley, a flashpoint

The security official said Ruto had left his home shortly before the attack to attend a campaign rally with Kenyatta. 

Ruto's home lies in Kenya's western Rift Valley area. The area was a flashpoint for postelection violence in 2007 that killed 1,100 people and ruined Kenya's image as a regional beacon of safety and stability.

Gideon Moi, son of former president Daniel arap Moi and an influential senator from the Rift Valley, issued a statement calling Saturday's assault "shocking and worrying."

"Kenyans at this moment want to peacefully participate democratically in electing their leaders and no criminal element or group should be allowed to jeopardise peace at this critical time," Moi said.

Recent opinion polls indicate this year's election will be tight, and tensions are rising.

Odinga has repeatedly accused the government of scheming to steal the election, while Kenyatta has accused Odinga of trying to delay the vote.

Earlier this month, Human Rights Watch said it received reports of threats and voter intimidation in Naivasha, a scene of violence in 2007 and one of the potential hotspots in this year's election.

Hate-speech flyers have been circulating in the Rift Valley, and some local residents have already left their homes.

The bloodshed of a decade ago haunted both Ruto and Kenyatta long after it ended. The International Criminal Court (ICC) put both men on trial for orchestrating the violence.

But the charges were subsequently dropped, with ICC chief prosecutor Fatou Bensouda blaming a relentless campaign of victim intimidation for making a trial impossible.

bik/tj (AFP, Reuters)