Two leading presidential hopefuls are among four prominent Kenyans whom the International Criminal Court ruled Monday should face trial over deadly post-election violence four years ago.

Kenya's President Mwai Kibaki appealed for calm in the east African country amid fears the Hague-based court's anxiously awaited decision could revive ethnic and political tensions.

The ICC said charges of crimes against humanity had been confirmed against William Ruto and Uhuru Kenyatta, the son of the country's founding president, dropping a bombshell in Kenya's presidential campaign.

Ruto dismissed as "strange" allegations against him over the violence that erupted following a disputed 2007 presidential vote, killing more than 1,100 people and shattering Kenya's image as a beacon of regional stability.

"After having thoroughly examined all the evidence presented by the prosecution, the chamber decided to confirm the charges against four of the six suspects," ICC presiding judge Ekaterina Trendafilova said.

"The chamber found that there are substantial grounds to believe Mr Ruto is responsible as an indirect co-perpetrator of the crimes," she said before a public hearing held at the court's headquarters.

"In regards to Mister... Kenyatta, the chamber was satisfied that the evidence established that there are substantial grounds to believe that (he is) criminally responsible of the alleged crimes, as an indirect co-perpetrator."

Two other Kenyans, radio host Joshua arap Sang, 36, and Francis Muthaura, 65, the head of Kenya's civil service, will also face trial.

Charges were dropped against former industrialisation minister Henry Kosgey and former police chief Muhammed Hussein Ali, the court said.

Nairobi had sought to pre-empt the impact of the ICC's decision saying that appeals were possible.

"I appeal to everyone to remain calm and peaceful. Our great nation has had its share of challenging times," Kibaki said Monday in a public statement.

ICC chief prosecutor Luis Moreno-Ocampo was allowed in March 2010 to investigate the six, three of them aligned with Kibaki's Party of National Unity and three who supported the opposition Orange Democratic Movement of Raila Odinga, now the prime minister in a coalition government.

Observers had feared the ruling could trigger fresh violence, four years after election fraud allegations sparked Kenya's worst unrest since independence and revealed deep ethnic rifts.

However the ICC's decision predictably struck a perfect balance between the two rival camps. Ruto and Sang were opposition supporters in 2007, while Kenyatta and Muthaura were and still are Kibaki allies.

"Allegations against me will forever be strange to me. My legal team will move expeditiously to analyse the ruling and the actions to take," Ruto told reporters, stressing he was still a presidential candidate.

A fresh presidential election in the east African country is due by March 2013 at the latest.

Kenyan and international legal organisations welcomed the ruling.

It is "an important step forward in ensuring justice for victims of the terrible crimes," said George Kegoro, president of Kenya's chapter of the International Commission of Jurists.

Kenyatta, also a deputy vice president, and Francis Muthaura, often described as Kibaki's right-hand man, are accused of attempting to keep the PNU in power "through any means necessary."

They face five counts including orchestrating murder, rape, forcible transfer and persecution in the polls' aftermath.

Kenyatta, 50, has been accused of ties with the shadowy criminal gang called the Mungiki -- a sect-like body known for skinning and beheading its victims -- which he allegedly directed to attack opposition supporters.

Ruto, 45, and radio boss Joshua arap Sang, 36, face three counts of murder, forcible transfer and persecution.

They are accused of "carefully orchestrating" attacks against ruling PNU supporters after Odinga accused Kibaki of rigging his way to reelection.

What began as political riots after the December 2007 vote soon turned into ethnic killings targeting Kenyatta's Kikuyu tribe.

This launched reprisal attacks in which homes were torched and people hacked to death in the worst outbreak of violence since Kenya acquired independence from Britain in 1963.