Prosecutor Luis Moreno-Ocampo told a press conference in The Hague, where the International Criminal Court is based, that investigators would examine "at least two cases and each case will have one to three individuals."
Speaking a day after the court authorised the probe, he said there was a confidential list of 20 suspects, but the focus had fallen "on those most responsible" for the killings.
"We aim to finalise the bulk of the investigations during 2010," he said.
The list of suspects was submitted by Kenya's own commission launched to probe the wave of violence that followed the December 2007 elections. The list includes political and business leaders linked to both the Party of National Unity of President Mwai Kibaki and the Orange Democratic Movement (ODM) led by Raila Odinga, which was then in opposition.
The two sides are now in an uneasy power-sharing government.
Moreno-Ocampo said that it had information indicating that both sides had taken part in "large-scale crimes."
"So far we lack elements of proof, we have only the information," he said.
He praised Kenya, which has pledged to cooperate with the court, for tackling the issue of the mass killings.
"Kenya will provide a historic example of how one country chose to address and prevent massive crimes," he said.
The International Criminal Court agreed to launch the probe in a written decision released Wednesday.
"The information available provides a reasonable basis to believe that crimes against humanity have been committed on Kenyan territory," the court statement said.
The ICC, which started operating in 2002 as the world's only permanent independent tribunal to try war crimes, crimes against humanity and genocide, can only take cases when countries are unwilling or unable to prosecute.
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