Solution: The diagonal of the rectangle is the diameter of the circumscribed circle. Using the Pythagorean theorem, the diagonal $ d = \sqrt10^2 + 24^2 = \sqrt100 + 576 = \sqrt676 = 26 $ km. The circumference $ C = \pi \times d = 26\pi $ km. Thus, the circumference is $ \boxed26\pi $. - Abu Waleed Tea