"We are ready for talks, but will never be reconciled with the annexation of our territory," Oleksander Turchinov said in a televised address to the nation.
Ukraine's Foreign Ministry earlier rejected a Russian proposal to create a contact group to mediate in the crisis and suggest changes to Ukraine's constitution.
In his address, Turchinov also said that any actions inciting mass disorder in Ukraine would been viewed as "abetting the military aggressor and a crime against the state".