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By Supantha Mukherjee and Stephen Nellis

Sept 20 (Reuters) - Apple Inc on Wednesday conceded its latest smartwatch unveiled a week ago has problems with its most important feature: the ability to make phone calls and access data without an iPhone nearby.

Several prominent reviewers said Wednesday they could not recommend the device because of a wifi glitch that causes cellular connectivity problems.

The Watch Series 3 starts at $399 and was launched alongside new iPhone models. Unlike previous versions of the watch, it has cellular network connectivity built in.

Apple said the watch can experience LTE connectivity problems when it connects to open wi-fi networks such as at a hotel or a coffee shop.

The company is "investigating a fix for a future software release," Apple spokeswoman Amy Bessette told Reuters.

Many reviewers such as the New York Times praised the new features and gave generally positive assessments.

But other prominent publications, including the Wall Street Journal and The Verge, recommended against purchasing the new model because the LTE cellular data connectivity did not work as expected.

The mixed reviews weighed on Apple shares, which closed down about 1.7 percent at $156.07.

"Considering that my Apple Watch Series 3 with LTE (both first and second review units) didn't function like it was supposed to, I can't recommend buying it — and paying the monthly cell fee — based on promises," Verge reviewer Lauren Goode wrote. (http://bit.ly/2fj8Jiy)

Apart from connection issues, some reviewers were disappointed with the drain on the watch's battery while making calls. Apple had touted up to 18 hours of battery life but said the watch would get only one hour on a cellular phone call. Gene Munster, a longtime Apple analyst with Loup Ventures, doubted issues with the Series 3 Watch would hurt Apple's bottom line. "That review takeaway is a negative but is not a surprise. This is the first generation watch with LTE," he told Reuters.

Bob O'Donnell, an analyst with Techanlysis Research, said the watch reviews, paired with reviews of the iPhone 8 that were generally positive but described the phone as an incremental improvement on its predecessor, put more pressure on the iPhone X to garner good press before its Nov. 3 launch.

Apple also experienced hiccups with iOS 11, the new operating system the firm released Tuesday.

For business users, iOS 11's Mail application had problems sending mail for Microsoft Exchange and Outlook.com mail accounts. (Reporting by Supantha Mukherjee in Bengaluru and Stephen Nellis in San Francisco; Editing by Anil D'Silva and Cynthia Osterman)