Apple's biggest iPhone maker Foxconn has been given the green light to resume production at a key plant in the eastern central Chinese city of Zhengzhou, after being forced to shut it following the coronavirus outbreak.

However, only 10 percent or 16,000 employees of the factory's workforce has managed to return so far, a source told Reuters.

With the shutdown and lack of hands in the plant, it is predicted that Apple's sales will fall by 10 percent this quarter.

However, the firm has not yet been allowed to restart production in Shenzhen, a southern manufacturing hub, the source said.

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Apple's biggest iPhone maker Foxconn has been given the green light to resume production at a key plant in the eastern central Chinese city of Zhengzhou (pictured), after being forced to shut it following the coronavirus outbreak

The two factories together make up the bulk of Foxconn's assembly lines for Apple's iPhones, and the delays are likely to impact global shipments.

Researcher TrendForce said on Monday it expected global smartphone production to fall by 12 percent to its lowest level in five years in the first three months of 2020 because a virus outbreak in China.

TrendForce said it cut its forecasts for the global smartphone output, expecting Huawei, Apple Inc. and other companies to bear brunt of the market slowdown - Apple is set for a 10 percent drop.

Apple itself gave a wider-than-usual revenue outlook range for the March quarter last month to factor in uncertainty due to the virus that has claimed more than 900 lives and infected over 40,000 people.

An Apple spokeswoman in Shanghai was not immediately available for comment.

About 16,000 people, or under 10 percent of Foxconn's workforce in Zhengzhou have returned to the plant, the source said, adding that company executives were trying very hard to negotiate with authorities to resume production in other parts of China

Apple rival and China's biggest smartphone maker, Huawei, said last week it had resumed production of consumer devices and carrier equipment, and operations were running normally.

About 16,000 people, or under 10 percent of Foxconn's workforce in Zhengzhou have returned to the plant, the source said, adding that company executives were trying very hard to negotiate with authorities to resume production in other parts of China, including Kunshan, in southeastern Jiangsu province.

'Our request to resume production (in Shenzhen) was disapproved. We need to improve our virus control measures for another check,' said the person who declined to be identified because they are not authorised to speak publicly on the matter.

Shenzhen authorities will conduct checks at the plant again later this week, the person said.

Employees in Shenzhen were told not to return to work on Tuesday, according to an internal memo seen by Reuters.

The coronavirus outbreak - declared a global health emergency by the World Health Organization - has disrupted Chinese manufacturing and forced companies such as Hyundai Motor to halt production of cars in some factories.

Some companies including Samsung Electronics limped back to work on Monday but hundreds of factories and stores remain shut across China.

Foxconn, formally Hon Hai Precision Industry Co Ltd, said in a statement that employee safety was top priority and that it was working with authorities to meet requirements to resume production across China 'in a staggered and orderly manner'.

Foxconn employees who returned to work on Monday following an extended Lunar New Year holiday have been told to wear masks, undergo temperature checks and adhere to a specified dining system, according to internal memos seen by Reuters.

With the shutdown and lack of hands in the plant, it is predicted that Apple's sales will fall by 10 percent this quarter

Most senior Taiwanese officials have been told to refrain from returning to China and those who needed to do so required approval from Chairman Liu Young-Way, the person said.

Foxconn, which makes devices for global electronics firms, has built its own production lines in the southern province of Guangdong to make masks for its hundreds of thousands of employees, targeting two million masks a day by late February, the memos showed.

The company reported an 11.96 percent drop in its January revenue from a year ago to T$364.6 billion ($12.12 billion), according to a Foxconn filing to the Taiwan stock exchange. It did not give further details.

Foxconn shares fell as much as 2.4 percent in Monday trade, lagging a 0.3 percent decline in the broader market.

They have fallen more than 11 percent since the market reopened following the Lunar New Year break.