The leaders of France, Germany, Ukraine and Russia agreed in a telephone call Thursday to make a new push to impose a tattered cease-fire in eastern Ukraine, a day after Kiev gave up a strategic rail hub to Russia-backed rebels following days of heavy fighting.

Kiev and its Western backers had denounced the separatists’ drive—which continued after the European-brokered truce was to have taken effect on Sunday—as a violation of the deal. But as fighting subsided along much of the front after the Ukrainian retreat from…