By Pavel Polityuk and Abdelaziz Boumzar KYIV/NIU-YORK, Ukraine (Reuters) -Ukraine ignored a Russian ultimatum to surrender the eastern city of Sievierodonetsk on Wednesday as the United States announced more weapons for Kyiv and urged its allies also not to “lose steam” in providing military support. Sievierodonetsk, now largely in ruins, has for weeks been the main focal point of the war. Russia had told Ukrainian forces holed up in a chemical plant there to stop “senseless resistance and lay down arms” from Wednesday morning, pressing its advantage in the battle for control of eastern Ukrain…