A top Croatian court has increased to six years the corruption sentence against former Prime Minister Ivo Sanader and ordered his immediate imprisonment.

Sanader was transferred to a Zagreb prison on Thursday evening after the court upheld prosecutors' appeal of the 2017 sentence of four and one-half years in prison for corruption in a 2009 property sale in Zagreb.

Sanader served as prime minister from 2003 to 2009. He is the highest-ranking official tried for corruption in Croatia.

The former premier has faced several corruption trials that are in various stages of proceedings. He has denied any wrongdoing, claiming that charges against him are political.

The court also ruled that Sanader must return over $2 million in gains from corruption.