NEW YORK — Walmart confirmed Thursday that it is closing dozens of Sam's Club warehouse stores — a move that a union-backed group estimated could cost thousands of jobs — on the same day the company announced that it was boosting its starting salary for U.S. workers and handing out bonuses.The world's largest private employer said it was closing 63 of its 660 Sam's Clubs over the next weeks, with some shut already. Ten are being converted into e-commerce distribution centers, according to a company official who spoke on condition of anonymity because he was not authorized to discuss details of the decision publicly.He said it was too early to say how many people overall would lose their jobs since some will be placed at other Walmart locations or rehired at the e-commerce sites. Making Change at Walmart, a campaign backed by the United Food and Commercial Workers International Union, estimates that 150 to 160 people work at each Sam's Club store, meaning the closures could affect about 10,000 people.Lauren Fitz, 22, said she was at her other job as a church secretary when a colleague texted to say that the Sam's Club where they both worked in Loveland, Ohio, had closed.