Apple Inc CEO Tim Cook is on a trip to China this week, pledging to boost investment in Shanghai and strengthen communication with local mobile app developers.

The move, amid the ongoing Sino-US trade frictions, underlines the importance of China, the world's largest mobile arena where Apple is locked in competition with rivals such as Huawei Technologies Co Ltd.

In a meeting with Shanghai Party Secretary Li Qiang on Tuesday, Cook said the US tech heavyweight will invest more in Shanghai and actively seek to expand cooperation, according to a report by Shanghai media.

Such comments came after US President Donald Trump asked Apple to relocate part of its iPhone plants to its home turf, in order to avoid his proposed tariff on Chinese imports. However, such a move could lead to an iPhone price rise of as much as 20 percent, experts estimated.

Currently, Apple has very strong ties with China where most of its manufacturing operations are located. The nation also accounted for around 18 percent of its total sales in the second quarter. Moreover, local developers are an essential part of Apple's ecosystem.

It has also proposed plans to establish a string of research and development centers in China in two years, including the latest initiative announced in March to create a joint research facility with Tsinghua University to focus on advanced technologies, including machine learning and computer vision.

But the US tech company is also facing a string of challenges, with Huawei and other local smartphone vendors working hard to erode its dominance in the premium segment.

Cook, however, remains optimistic, saying he has never been as hopeful regarding the China market as he is now. He added in an interview with news portal Sina.com that the company's prowess in integrating software and hardware will give it an upper hand.

During his trip, which started on Monday, Cook visited a number of local mobile app developers and users who are using Apple products.

Such frequent communication highlights Apple's recognition of China's innovative capabilities and the importance of Chinese developers to the tech giant, said Zhao Ziming, an analyst at Beijing-based consultancy Analysys.

Apple said there were around 1.8 million iOS Chinese app developers on its ecosystem last year.

On Wednesday, Alipay, China's largest mobile payment platform, made the rare move of warning that its users making payments via Apple devices may face a risk of money loss due to possible security problems. Apple has so far not responded to requests for comment.