By Catherine Belton LONDON (Reuters) -For NATO members, the most powerful measure against Russia were it to invade Ukraine would be U.S. sanctions cutting off Russian state banks from the dollar according to Russian executives, bankers, and former senior U.S. sanctions officials. The United States has warned that Russia could invade as early as this week. Moscow denies it has such plans but says the West needs to take its concerns about NATO expansion seriously. Washington, and its allies in Europe, are finalising an extensive package of sanctions if Russia were to launch an invasion according…