"On the occasion of this grave natural disaster which has claimed the lives of many US citizens and caused huge property damage, please accept my condolences and the condolences of all Croatian citizens," reads the letter.
Hurricane Katrina has left hundreds of casualties in its wake and flooded many areas, including the historical city of New Orleans.
The US authorities have launched the biggest mobilisation in the history of the country, sending hundreds of government trucks, Coast Guard planes and rescue teams to Mississippi, Louisiana, Alabama and Florida, the four states most affected by the hurricane.
The damage caused by the hurricane is estimated at 26 billion dollars.