By Pavel Polityuk and Simon Lewis KYIV/KRAMATORSK, Ukraine (Reuters) – Ukraine has so far staved off any major Russian advance into the north of its Donetsk region, but pressure is intensifying with heavy shelling, missile and rocket fire on the city of Sloviansk and nearby populated areas, Kyiv’s military said on Wednesday. Russia and separatist proxies were already in control of the southern part of Donetsk province when they effectively completed the seizure of the neighbouring Luhansk region on Sunday with the capture of Lysychansk, much of which now lies in ruins. Moscow says ejecting the…