Investors will be watching how this impacts U.S. technology firms. Here's what exposure they have to China.

It follows tariffs announced on Friday of 25 percent on up to $50 billion of Chinese goods, including semiconductors.

Apple could be most at risk of all the big American technology firms if the trade tensions between the U.S. and China escalate.

"Apple is most exposed," Neil Campling, co-head of global thematic group at Mirabaud Securities, told CNBC by phone Tuesday.

Drilling into the numbers you can see why. In its last fiscal year, Apple generated nearly 20 percent of its revenues from Greater China, which equated to $44.7 billion. In 2017, it shipped over 41 million iPhones into China and was the fifth-largest player in the market, according to data from IDC.

On top of that, it has around 40 stores in China. And it also operates its services such as the App Store and Apple Music in China. Services are an increasing part of Apple's business as the smartphone market slows down; in the 2017 fiscal year, they accounted for 13 percent of total net sales, up from 11 percent in 2016. Apple does not break down how much of this came from China.

Apple also relies heavily on Asian suppliers. Its iPhones are assembled in China by Taiwanese firm Foxconn.

Arguably, Apple has become the most successful U.S. technology company in China but that same strength could be a vulnerability.

While Apple CEO Tim Cook reportedly got assurances from President Trump that iPhones assembled in China would not be subject to tariffs, there are still risk should a trade war escalate.

One risk is that Beijing clamps down on Apple's suppliers, causing delays or even raising concerns about Apple products. The New York Times reported, citing a person close to the company, that this is a concern within Apple should the government do this under the guise of national security.

Authorities could also ban Apple services, which it has done before. In 2016, Apple's iBooks Store and iTunes Movies were shut down in China.

Beijing could also push its own homegrown smartphone companies, like Xiaomi and Huawei. The latter faced scrutiny in the U.S. with intelligence officials cautioning U.S. consumers not to buy Huawei phones for fear of being spied on by the Chinese. China could theoretically adopt a similar message, saying the iPhone is a danger to national security.

Campling noticed that Apple's inventories, which totaled $4.4 billion in the three months ending December 30, jumped to $7.6 billion in the March quarter. Inventories include completed products as well as components used in Apple devices. He said this was evidence that Apple was stockpiling components in case of any disruption and showed the company was concerned.

"It is a defensive/protective measure in case there are difficulties in future procurement or supply chain disruption as Apple is potentially in the crossfire of the U.S./Sino trade war," Campling wrote in a note to clients Tuesday.