Boeing said on Tuesday that it had withdrawn its pay offer to around 33,000 U.S. factory workers and no further negotiations were planned with their union representatives as a financially damaging strike nears its fourth week. Boeing and the union held their latest round of negotiations with federal mediators on Monday and Tuesday, but talks collapsed and the sides were left locked in acrimonious stalemate showing no signs of being resolved anytime soon, a person briefed on the talks said.