US Secretary of State Condoleezza Rice is to go to Kenya amid efforts to end the violence that erupted after a disputed presidential election.
Ms Rice is travelling with Mr Bush on his tour of Africa
President George W Bush said Ms Rice would back the mediation efforts led by former UN chief Kofi Annan.
She would press for an end to violence, justice for victims, and a return to democracy, Mr Bush said.
At least 1,000 people have been killed and more than 600,000 displaced during Kenya's post-election unrest.
Mr Annan has been pushing for an agreement between Kenyan President Mwai Kibaki, who claimed victory in December's election, and opposition leader Raila Odinga, who has argued that the poll was rigged.
There were reports on Thursday that the two sides had reached an agreement in principle to form a grand coalition.
Talks are due to resume next week.
Ms Rice will be accompanying President Bush on his trip to the African states of Tanzania, Rwanda, Benin, Ghana, and Liberia, which begins on Saturday.
"When we are on the continent, I have asked Condi Rice to travel to Kenya to support the work of the former secretary general and to deliver a message directly to Kenya's leaders and people," President Bush said.
"There must be an immediate halt to violence, there must be justice for the victims of abuse and there must be a full return to democracy."
Meanwhile, Kenya's government has ordered an investigation into claims that radio stations broadcast hate speeches to fuel ethnic violence.
Information Minister Samuel Poghiso said that politicians who had used hate speech would face the law.
BBC World News America presenter Matt Frei will be interviewing President Bush later on Thursday. The full video and transcript will appear on this website.