Slovakia's Prime Minister Robert Fico has accused the country's president of destabilising the nation after the killings of an investigative journalist and his fiancee.

Police are conducting raids in eastern Slovakia on houses linked to members of Italian mafia, after it emerged a journalist was writing a story about them before he was shot dead last week.

A journalist shot dead in Slovakia was working on a story about the activities of Italian mafia in his country and their links to people close to Slovakian Prime Minister Robert Fico.