LOS ANGELES (Reuters) - Twenty-First Century Fox cannot buy publisher Tribune Co or its Los Angeles Times newspaper, Fox Chief Executive Rupert Murdoch wrote on Twitter on Friday, citing U.S. laws on media ownership. "Sorry can't buy Trib group or LA Times - cross-ownership laws from another age still in place," said tweets attributed to Murdoch. Fox earlier this week revealed it had made a roughly $80 billion bid for Time Warner Inc that was rebuffed. (Reporting by Lisa Richwine; Editing by Bernard Orr)