WASHINGTON (Reuters) -U.S. manufacturing moved closer to recovery in December, with production rebounding and new orders rising further, but the outlook remains uncertain amid the threat of higher tariffs that could raise prices of imported raw materials. Despite the increase in the Institute for Supply Management's (ISM) Purchasing Managers Index (PMI) to a nine-month high last month, the tone of the survey was less upbeat, with phrases such as "volume decreases" and "significant slowdown" appearing in some of the comments from respondents. None of the six largest manufacturing industries grew last month.