Any provocation or aggression against Ukraine would have serious consequences, EU foreign affairs chief Josep Borrell said in a message to Moscow ahead of a meeting with Ukrainian officials on Wednesday.
"The European Union is Ukraine's closest partner and we fully support Ukraine's territorial integrity and sovereignty," the top EU diplomat said in an interview with Polish news agency PAP that was published on Tuesday.
During his visit, Borrell is to meet with Ukrainian officials including Foreign Minister Dmytro Kuleba in Kiev and in eastern Ukraine.
Borrell's efforts are part of an international push to calm a brewing crisis at the Ukrainian border.
For weeks, the West has been concerned about reports that Russia has massed tens of thousands of soldiers in areas not far from Ukraine, triggering fears of a Russian invasion.
Russia-US talks in Geneva
Russia rejects the charge, and in turn accuses Ukraine of having moved more soldiers to the eastern areas held by pro-Russian separatists.
The developments recall the events of 2014, when Russia annexed Ukraine's Crimean Peninsula and began its ongoing support for separatists in eastern Ukraine.
Talks between the United States and Russia are to be conducted in Geneva on Monday. Russia and NATO are planning separate talks two days later, on January 12.