"I believe that the sinking of the ship was a premeditated act carried out by North Korean forces," Ramos-Horta said in a press statement.
"East Timor most strongly condemns this unprovoked, hostile and unlawful military action," the statement said.
South Korea officially blamed North Korea on Thursday for the sinking of the South Korean warship Cheonan that killed 46 sailors. North Korea has denied any involvement in the incident on March 26.
Ramos-Horta said the North Korean government is an aberration in the 20th and the 21st centuries.