WASHINGTON (Reuters) - The port city of Mariupol in eastern Ukraine was still contested as Russian appeared to have sent reinforcements into Ukraine in recent days, a senior U.S. defense official said on Monday.

       "Our assessment is Mariupol is still contested... (it) remains under threat from the air but both from missile strikes as well as bombs from the air but even of course artillery," the official, speaking on the condition of anonymity, told reporters.

       The official said there were roughly 76 Russian battalion tactical groups in southern and eastern Ukraine currently, an increase of about 11 in recent days.

       (Reporting by Idrees Ali and Kanishka Singh.)