By Pavel Polityuk and Natalia Zinets (Reuters) – The southern Ukrainian port of Mariupol has been under heavy fire from Russian forces since soon after the war began on Feb. 24. Nearly 5,000 people have been killed and buildings across the city destroyed, according to the mayor. He said 290,000 people had fled by March 27 but that at least 160,000 were still trapped without power and with little food or other supplies. Why is the city so important? STRATEGIC LOCATION Mariupol is the biggest Ukrainian city on the Sea of Azov, which is connected to the Black Sea by the narrow Kerch Strait. The c…