Dec 18 (Reuters) - CNN is closing its offices in the United States to all nonessential employees as COVID-19 cases increase, the network said on Saturday in an internal memo to staff seen by Reuters. CNN, part of AT&T Inc's (T.N) WarnerMedia division, will close its offices to all employees who do not have work in the office, the memo said.... ...The news was first reported by the Wall Street Journal. The network had set a tentative return-to-office date in January and it isn't known if that date will move, the Journal reported....