Our daughter who lives in the city had a similar situation, the pit bull finally broke the fence boards and then broke into the coop and killed all 3 chickens. It turned out the neighbors' landlord didn't allow pit bulls, and he evicted them, fixed the fence, and now the new chickens have a secure pen. Definitely report to animal control. It is illegal to have a dog at large, and in most areas, illegal for dogs to kill livestock as well. Do what you can to protect the chickens before it's too late!