European leaders on Thursday stood by the cease-fire they brokered a week ago for Ukraine, even as the U.S. said Russian equipment and troops continued to flow into the country and the Ukrainian military significantly raised its latest casualty toll.

Kiev and its Western backers had denounced a dayslong offensive by Russia-backed separatists to seize the strategic rail hub of Debaltseve as a violation of the cease-fire. But as fighting subsided generally after the hasty retreat on Wednesday by Ukrainian forces, none of the...