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Pressure on Apple Inc. in China intensified as a Shanghai consumer group loosely affiliated with the government complained of additional problems with batteries in the company's iPhone 6 devices.

Shanghai's Consumer Council said it received eight reports from users claiming their iPhone 6 series devices had spontaneously caught fire. It marked the third battery-related complaint against Apple from Chinese consumer groups over the past month.

Apple said it analyzed the affected phones and found that the fires followed "external physical damage." The company encouraged customers with issues to visit an Apple store or contact company support.

"We appreciate that customers are more concerned than ever about the performance and safety of batteries in their mobile devices," Apple said in a statement.

The pressure from Chinese consumer groups comes at a critical time for the company. Sales of iPhones in Greater China soared in 2014, helping Apple establish itself as the world's most profitable company. But rising competition from homegrown rivals has cut into Apple's growth in the region. Sales to Greater China, which includes Hong Kong and Taiwan, fell 17% in the fiscal year ended Sept. 24 after growing 84% in the prior year.

In September, Apple rival Samsung Electronics Co. recalled millions of Galaxy Note 7 devices because of user reports that the phones were catching on fire or exploding while charging.

Apple has repeatedly said its battery issues aren't related to safety. The majority of the complaints involve iPhone devices unexpectedly shutting down even when half their battery life remained.

The battery-life issue was first highlighted in mid-November, when the China Consumers Association said consumers were having trouble with the iPhone 6 and iPhone 6s.

Apple said a company investigation found that the problem was limited to iPhone 6s devices made in September and October 2015 whose battery components were exposed to "ambient air" longer than they should have been during assembly. Apple said it would replace the faulty batteries world-wide.

Tuesday, Apple said it also would update its iOS software that runs iPhones next week with a "diagnostic capability" to gather more information about battery-related issues. The company said it could use the information to "improve the algorithms used to manage battery performance and shutdowns."

The complaints from the consumer groups follow separate Chinese government moves to rein in Apple's patents and shut down Apple's online book and movie services because they violated local media guidelines.

Apple's troubles in China come amid growing concerns about U.S.-China relations after the election of Donald Trump, who last week accused China of currency manipulation. He also broke diplomatic protocol last week by speaking by phone with Taiwan President Tsai Ing-wen.

Mr. Trump has criticized Apple for making the vast majority of its products in China.

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

(END) Dow Jones Newswires

December 06, 2016 13:25 ET (18:25 GMT)

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