REPUBLIKA.CO.ID, MOSCOW -- Russia's Foreign ministry said on Friday it was deeply disappointed US President Barack Obama had signed into law a new bill on sanctions against Moscow over the Ukraine crisis.

"We do not leave hostile actions by Washington without a reaction," it said in a statement. "Decisions on a possible response will depend on how the US use the new law in practice."

Earlier on Thursday, European Union leaders warned Moscow they are ready to flex their combined muscle and "stay the course" in a long confrontation with Russia if President Vladimir Putin refuses 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.

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.

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