Allan Aliali, a journalist for the Notre Voie newspaper of the president's party, told UN radio: "In the prime minister's cabin I could see three dead bodies that I could not identify."
Soro was named prime minister in April after signing a peace deal with Laurent Gbagbo, the president, but have said that some factions in his New Forces group had branded him a traitor.
The New Forces rebels seized the north of the West African country during a brief 2002-2003 civil war
A Reuters news agency reporter later saw Soro arrive unharmed at the group's headquarters in the town, although other members of his entourage had visible injuries.
According to a Soro aide, shooting was heard in Bouake following the attack.
Bernard Kouchner, the French foreign minister issued as statement saying: "France strongly condemns this cowardly attack and reaffirms its solidarity with the whole Ivory Coast nation and people."
France, which ruled Ivory Coast until independence in 1960, has troops stationed in the country to support a United Nations peacekeeping force.