Polar bears can have furtile offspring with grizzly/brown bears. See: https://en.wikipedia.org/wiki/Grizzly%E2%80%93polar_bear_hyb...
in general, there are multi organizational levels of reproductive incompatability.
in this case, the geographic distance, orogenic blockade, and ecological confounds of arctic conditions preclude easy mingling of U.arctos x U. maritimus.