Nikon China is under investigation for allegedly selling defective D600 cameras and failing to address customer complaints.

The state broadcaster CCTV highlighted the business misconduct in an annual program on March 15, in conjunction with the celebration of the World Consumer Rights Day.

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Customers had complained about dust buildup on the image sensor of the Nikon D600 camera.

According to the CCTV report, a person had complained that the image sensor of his D600 camera has been cleaned five times but dust continued to accumulate.

Du Yang, another Nikon D600 camera owner, was shown during the March 15 event using his camera which has been cleaned at the Nikon service center in Shanghai.  After 10 shots, the dust came back. Cleaned for a second time, but the dust came back after taking some shots.

"We regard this matter very seriously ... and express sincere apologies to our customers," Nikon said in a statement.

"We thank the Chinese government, media and consumers for such close attention and supervision over Nikon," the statement added.

Nikon also said that even if the original warranty period had ended, it would still accept D600 cameras for repair and maintenance.

Nikon D600 camera sales were halted on shopping websites JD.com and Suning.com on Sunday and replaced the model with the D610 model.

On taobao.com, China's largest online shopping website, D600 cameras were still available as of Sunday afternoon.

Under China's consumer rights law, the owner of a digital camera has the right to a refund or replacement if the product still doesn't work after it has been repaired by the manufacturer twice.

But the CCTV report claimed that Nikon turned down the request of D600 users, including Du, to get a refund or have their units upgraded to the newer model D610 for free.

By contrast, some D600 owners in the United States were reportedly offered a free upgrade to the D610.

The Shanghai Municipal Bureau of Industry and Commerce is investigating the issue raised by the CCTV report.

The local government will conduct the investigation on the complaints before taking further measures, Shi Shulu, deputy director of the bureau's Consumers' Rights Protection Department said.

In its micro blog, Nikon China said the company attaches great importance to the report and will give the Chinese customers high quality and standard global service.

Nikon issued an advisory that all D600 owners are eligible to a free inspection, cleaning and replacement of shutter assembly and related parts.

CCTV noted, however, that Nikon has failed to reveal the real cause of the problem, and has yet to comment on the issue to the frustration of D600 owners.