European Union leaders warned Moscow they were ready to exercise their combined muscle over the long haul in a confrontation with an economically wounded Russia if President Vladimir Putin refused to pull back from Ukraine.

"We must go beyond being reactive and defensive. As Europeans we must regain our self-confidence and realize our own strengths," said Donald Tusk, the former Polish premier who chaired a brief EU summit in Brussels on Thursday.

In comments that were part warning to Russia, where falling oil prices and Western trade sanctions have brought financial havoc, and part exhortation to an EU bloc divided between hawks and doves, Tusk said a united European front was vital.