Ukraine's main strongholds in the Donbass region are coming under Russian attack from four separate directions. At least 12 people have reportedly been killed in Severodonetsk, and dozens more wounded by Russian shelling. British intelligence says the city has become one of Moscow's "immediate tactical priorities". President Volodymr Zelenskky says Ukraine could lose up to 100 soldiers a day defending the east. Laurie Timmers explains the importance of the Donbass.