Wal-Mart announced on Thursday it would lift its minimum U.S. wage to $9 an hour this year and $10 in 2016, raising the ante in a tightening labor market where low-skilled workers easily move between retailers and fast-food chains. Labor and shareholder activists will likely use the news to ratchet up pressure on companies which have so far faced less scrutiny than Wal-Mart, the largest private U.S. employer with a workforce of 1.3 million.