A crowded open-air market in Ukraine's strategically important coastal city of Mariupol came under rocket fire this morning, killing at least 10 people, regional police said. Mariupol lies on the Azov Sea and is the major city between mainland Russia and the Russia-annexed Crimean Peninsula. Heavy fighting in the region in the autumn raised fears that Russian-backed separatist forces would try to establish a land link between Russia and Crimea.