"They are Mungiki members who started resisting arrest when police launched the operation to recover firearms," he said.
The two slain policemen had been carrying out a routine patrol in the Mathari slums of northern Nairobi when they came under heavy gunfire from suspected Mungiki members, he said.
The politically-linked Mungiki sect has been blamed for a wave of recent murders including several gruesome beheadings.
The religious group, with alleged historic ties to the Mau Mau independence uprising, comprises mainly of snuff-taking, dreadlocked youths which supports traditions such as female circumcision and oath-taking.
|"The operation will not stop until all the firearms they stole from the police are recovered"|
Slum residents said there were scenes of chaos as heavily-armed riot police sought out Mungiki members on Monday evening.
"I just heard gunshots and I could not tell what was happening. I could not go out to check, but I could hear the noise of people screaming," Kennedy Mwaura, a resident, said.
The group - notorious for criminal activities including extortion, murder and harassment of women - was banned in 2002 following deadly violence.
"I was beaten by five policemen. They didn't want to know whether I was Mungiki or not," Peter Njenga, a resident, said.
"They hit me with machetes and clubs. They told me to show them where the Mungiki are, but I don't know them."
Others also accused police of heavy-handedness and several women reported rapes.
"The police came, broke down doors and arrested men, then raped women. One of my daughters was raped," Mama Njeri said.
As others frantically searched mortuaries and police stations for missing relatives, Kiraithe said the crackdown was continuing.
"The operation will not stop until all the firearms they stole from the police are recovered. But so far, we have have recovered three pistols, six rounds of ammunition and 15 machetes," he said.
Meanwhile, a senior police official said that Mungiki members killed at least four people in Karuro town - about 80km northeast of the capital - in the early hours of Tuesday.
Hundreds of paramilitary police then patrolled the area seeking Mungiki members.
Mwai Kibaki, Kenya's president, vowed on Friday to crack down on the Mungiki, after it was blamed for the killings of five people, including one in his own constituency.
"We will not allow criminals to get away with wanton acts of violence," Kibaki said.
Many Kenyans believe corrupt politicians and police officers have been in league with the group.