By Doina Chiacu and James Oliphant WASHINGTON (Reuters) -The United States and NATO on Sunday condemned Russian President Vladimir Putin’s order to put his nuclear forces on high alert as dangerous and unacceptable, while the White House said it has not ruled out imposing new sanctions on Russia’s energy sector. In issuing the order to prepare Russia’s nuclear weapons for increased readiness for launch, Putin cited “aggressive statements” from NATO allies and widespread sanctions imposed by Western nations. The U.S. ambassador to the United Nations, Linda Thomas-Greenfield, told CBS’s “Face th…