(Adds WHO statement, paragraphs 10-15)

GENEVA, Feb 1 (Reuters) - Increasing numbers of Kenyan women and children are raped nightly in displacement camps, where sexual violence is used to threaten and intimidate, United Nations agencies said on Friday.

Elisabeth Byrs of the U.N. Office for the Coordination of Humanitarian Affairs in Geneva said there had been 167 rapes reported to the Nairobi Women's Hospital in the past month.

The youngest victim was 1 year old.

"Aid workers feel that (the) number of cases reported is only a fraction of the actual caseload," Byrs told a news briefing.

It is unclear who is committing the attacks, she said. The rate of reported rapes has more than doubled since the beginning of Kenya's post-election violence that has killed 850 people.

About 290,000 Kenyans have been uprooted by the riots and turmoil since the disputed Dec. 27 presidential vote, Byrs said, citing the latest Kenyan Red Cross figures.

She said the increasing sexual attacks reflected in part a collapse in the social order in Kenya, where the re-election of President Mwai Kibaki has exposed decades-old divisions between ethnic groups over land, wealth and power.

Rape is also being used as a tool to threaten and intimidate different communities, Byrs said.

"Perpetrators are exploiting the conflict to commit sexual attacks with impunity," she said, noting that few women were seeking medical attention after attacks, raising the risks of HIV and other sexually-transmitted infections.

The World Health Organisation also cited concerns over "dramatic increases" in sexual attacks in Kenya's Rift Valley.

"In many settings, survivors have no access to even the minimum health and psychological support, leaving them vulnerable to a range of potential negative health problems," the Geneva-based agency said.

In a statement, the WHO also raised concerns about the overcrowded sites hosting displaced Kenyans, often without reliable clean water. In some camps, there is only one toilet per 500 people, while food and cooking fuel is scarce.

Hospitals and health clinics in the region are also facing shortages of antibiotics, malaria medicines and drugs for chronic illnesses including asthma and diabetes, the WHO said.

It reported a concerning rate of diarrhoea in children as well as acute respiratory infections, and said many HIV and tuberculosis patients were struggling to access their medicines regularly, adding to concerns about the health of the displaced.

"Counselling services are not available, including for reproductive health, sexual violence or HIV/AIDS," the WHO said. (Reporting by Laura MacInnis; Editing by Caroline Drees)

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