WASHINGTON (Reuters) - The United States has been engaging in back-channel diplomacy with North Korea for several months, the Associated Press reported on Friday, as President Donald Trump stepped up his warnings to Pyongyang amid rising tensions in the region. The dialogue involved Joseph Yun, the U.S. envoy for North Korea policy, and Pak Song Il, a senior North Korean diplomat at the United Nations, the AP reported, citing unidentified U.S. officials and others briefed on the process.      