You can gain strength while dieting, but you can not increase the muscle mass in normal circumstances. Gaining muscle requires a surplus of calories and lose fat you need a calorie deficit, it is better to focus on one thing at a time. This is why bodybuilders have cut and collection phases. I want to spend the next regime 2 or 3 months and then focus on muscle mass for about a year. To increase muscle mass, do not try to win over a pound a week.