WASHINGTON (Reuters) - U.S. Secretary of State Rex Tillerson on Thursday welcomed Chinese efforts toward North Korea amid tensions over Pyongyang s nuclear weapons program, but said Beijing could do more with its oil exports to pressure North Korea.    The Chinese are doing a lot. We do think they could do more with the oil. We re really asking them to please restrain more of the oil, not cut it off completely,  Tillerson said in remarks at the State Department. 