WASHINGTON (Reuters) – The United States is expected to provide an additional $450 million in security assistance to Ukraine, including more long-range rocket systems, two U.S. officials told Reuters on Thursday. Moscow’s forces are advancing in Ukraine’s east in a bid to capture the industrial heartland known as the Donbas, where Ukraine fears some of its troops could be encircled in a Russian pincer move. One of the officials, speaking on the condition of anonymity, said an announcement was expected later on Thursday and the latest package is expected to include four additional High Mobility…