European leaders stepped up pressure on Russia to accept an unconditional cease-fire in Ukraine, threatening to immediately impose a new round of punishing sanctions if the Kremlin did not change its stance by the end of Monday. “The clock is ticking — we still have 12 hours until the end of this day,” the German government spokesman, Stefan Kornelius, told a news conference. On Monday, the Kremlin spokesman brushed off the threat. “The language of ultimatums is unacceptable — you cannot talk to Russia like this,” the spokesman, Dmitri S. Peskov, told Russian news agencies.