The premier said the "entry of Russian troops into Ukraine" is "an intolerable escalation which will lead to very serious consequences".
Renzi also expressed his "major concern" over the Ukraine crisis and "called for a swift return to the negotiating table", according to the statement issued by his office.
The Italian leader, whose country currently holds the rotating presidency of the EU, had met in recent days with Ukraine's President Petro Poroshenko. The two men will meet again during an EU summit in Brussels on Saturday.
Italian Foreign Minister Federica Mogherini meanwhile spoke to her Ukrainian counterpart Pavel Klimkin to assure him that the crisis would be at the centre of discussions during an informal meeting of EU foreign ministers on Friday in Milan.