A senior International Criminal Court (ICC) judge has warned Kenyan media and bloggers not to reveal the identity of witnesses at the trial of Deputy President William Ruto.
The first prosecution witness, who gave evidence on Tuesday, was not named.
But the BBC's Odhiambo Joseph in Nairobi says a local newspaper has published the photograph of a woman it claimed to be the witness.
Mr Ruto denies organising violence after the December 2007 elections.
He is charged with crimes against humanity over the violence, in which some 1,200 people died and 600,000 were forced from their homes.
ICC chief prosecutor Fatou Bensouda has previously complained that some witnesses were being intimidated in Kenya, some of whom have withdrawn from the case.
Her first witness gave evidence from behind a curtain and with her face pixelated and voice distorted on the court video.
She is being referred to as witness 536.
Our correspondent says that, after her appearance, numerous bloggers and social media entries have published her supposed name.
"Now that a witness has been identified it will be difficult to assure others that they and their family members will be safe. And in Kenya, it's not just the nuclear family: There are aunts, uncles, cousins," the AFP news agency quotes her as saying.
"Witnesses are bound to feel that their family and their extended family may be in danger and might want to withdraw," she said.
The proceedings are being closely followed in Kenya.
Presiding Judge Chile Eboe-Osuji warned that anyone revealing the identity of a protected witness could be guilty of contempt of court.
Our reporter says that the ICC does have jurisdiction in Kenya, but the situation is complicated as it would rely on political good will.
Furthermore, parliament recently voted to withdraw from the ICC, which would end the court's jurisdiction in the country.
Witness 536 broke down in court during her testimony on Tuesday.
She is said to have survived the attack on the Kiambaa Church in which about 36 people were burnt to death.
She is a member of the Kikuyu ethnic group, who were targeted by Mr Ruto's Kalenjin community, allegedly at his behest.
Mr Ruto is the first serving official to appear at the ICC.
President Uhuru Kenyatta is due to start his trial in November - he too denies the charges.