A number of modern maps, such as this one, show the Mongol Empire's northern border coming within ~500 km of the Arctic Ocean, near modern-day Arkhangelsk. The Ainu, a Japanese ethnic group, travelled to Kamchatka before the arrival of Russian explorers.
This begs the question of whether East Asia was aware of the Arctic Ocean before the arrival of European maps. Is there any evidence that the Mongols sent an expedition to the Arctic coast, or brought news of Viking and Russian discoveries to the East? Alternatively, did trade and cultural exchange with Siberian peoples provide information about the Arctic Ocean?
Let's say any mention of a northern sea, that is unambiguously not the Bering, Okhotsk, or North Atlantic, qualifies.