Mariupol, a key port city in southern Ukraine, has been under siege for days, leaving civilian areas burning and destroyed, according to video verified by The New York Times and interviews with residents. Images have emerged only sporadically because of power and cellular network outages in the city. Mikhail Vershinin, head of the Donetsk Regional Patrol Police, told The Times that once residents had restored their connections, he expected “horrible footage” to flood the internet. He described the situation as being on “the brink of a humanitarian disaster.”