A Chinese consumer-protection group said Tuesday it is investigating complaints that some Apple Inc. iPhones are spontaneously shutting off even when half their battery life remains.

The problem, which reportedly affects iPhone 6 and iPhone 6s devices, is the latest challenge for Apple in China, where sales have slowed and it has brushed up against a more assertive government. The announcement also comes amid growing concerns about U.S.-China relations after the election of Donald Trump, who has described China as a currency manipulator.

The China ConsumersAssociation, which is state-approved but isn't an official regulator, said users have reported their iPhones shutting down automatically despite having more than 50% of battery power left. The phones can't be turned on again until they are plugged in?even after a system upgrade, the association said.

The consumer group said it has approached Apple about the issue and would continue to monitor it.

An Apple spokeswoman had no immediate comment.

Apple has invested heavily in China in recent years. Chief Executive Tim Cook has traveled more to China than any other country over the past three years and said that China will eventually surpass the U.S. as Apple's largest market.

But Apple sales, after rocketing higher, are falling in China as the company faces more competition from homegrown rivals. Revenue in Greater China, which includes Hong Kong and Taiwan, fell 17% in the fiscal year ended Sept. 24, after growing 84% in the prior year.

The consumer investigation follows two Apple brushes with Chinese regulators earlier this year. In June, Beijing's municipal intellectual-property bureau awarded an injunction against sales of iPhone 6 and iPhone 6 Plus devices in a patent case. The injunction hasn't been implemented because Apple has appealed. In April, the national government shut down Apple's online book and movie services because they violated local media guidelines.

The complaint by the consumer group evokes memories of incidents in 2013 and 2014, when Apple was accused by state-run China Central Television of skirting warranties on iPhones and tracking user locations. China sometimes uses state-run media and semiofficial sites to signal permissible, and impermissible, behavior to foreign companies.

At the same time, Mr. Trump's criticism of China as a currency manipulator has stoked fears of a trade war and backlash against U.S. companies like Apple that depend on China for a sizable portion of total sales.

Eva Dou contributed to this article.

Write to Tripp Mickle at Tripp.Mickle@wsj.com

(END) Dow Jones Newswires

November 15, 2016 18:25 ET (23:25 GMT)

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