LONDON — A Russian-backed separatist official said that at least 1,200 civilians, including 127 children, are being “held hostage” at a chemical plant in the besieged Ukrainian city of Severodonetsk.Speaking to the Russian state news agency TASS, the pro-Russian Luhansk People’s Republic’s interior minister, Vitaly Kiselev, alleged that Ukrainian forces were keeping civilians at the Azot plant “against their will.” “They have been held there against their will for quite a while,” he said on Wednesday, according to an English translation of the news release. “There is no water, no food, no medicines.” According to the official, it is believed...