By Pavel Polityuk and Abdelaziz Boumzar KYIV/SLOVIANSK, Ukraine (Reuters) -Ukrainian troops holding out in the ruins of Sievierodonetsk came under renewed heavy assault on Wednesday from Russian forces who see the capture of the industrial city as key to control of the surrounding Luhansk region. In southern Ukraine, another major battleground in the war, authorities said Russian attacks on agricultural sites including warehouses were compounding a global food crisis that has stirred concerns of famine in some developing countries. Turkey hosted Russia’s foreign minister to discuss a U.N. plan…