By Steve Holland and Jeff Mason WASHINGTON (Reuters) -The United States will impose additional sanctions next week against Russian private military company the Wagner Group, which U.S. officials say has been helping Russia’s military in the Ukraine war, the White House said on Friday. White House national security spokesperson John Kirby said the U.S. Treasury Department will designate Wagner as a significant Transnational Criminal Organization. “In coordination with this designation, we will also impose additional sanctions next week against Wagner and its support network across multiple cont…