UNITED NATIONS — The U.N. Security Council on Wednesday unanimously approved the toughest sanctions on North Korea in two decades, reflecting growing anger at Pyongyang’s latest nuclear test and rocket launch in defiance of a ban on all nuclear-related activity. The United States and China, North Korea’s traditional ally, spent seven weeks negotiating the new sanctions, which include mandatory inspections of cargo leaving and entering North Korea by land, sea or air; a ban on all sales or transfers of small arms and light weapons to Pyongyang; and expulsion of diplomats from the North who engage in “illicit activities.” The U.S., its Western allies and Japan pressed for new sanctions that went beyond the North’s nuclear and missile programs but China was reluctant to impose measures that could threaten the stability of the neighboring country and cause its economy to collapse.