Trump reportedly promised China's president that he'd stay quiet on Hong Kong protests as long as trade talks progressed

• President Donald Trump promised Chinese President Xi Jinping in a June that he would stay quiet on the protests in Hong Kong in exchange for progress in the trade war, CNN reported Thursday.
• The president has so far only offered a muted response to the months-long protests, despite global backlash over Chinese encroachment on the semi-autonomous city.
• Details of Trump's call with Xi followed comments made by Trump on Thursday, in which he suggested that China should investigate former Vice President Joe Biden. 
• Trump's phone calls with foreign leaders are at the center of a whistleblower complaint, which has sparked a formal impeachment inquiry.
• US-China trade negotiations remain deadlocked.
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President Donald Trump has offered a muted response to the months-long protests that have rocked Hong Kong since June, and Trump's trade war with China may have been the reason. 

In a June phone call with Chinese President Xi Jinping, Trump mentioned the political prospects of his Democratic rivals, former Vice President Joe Biden and Sen. Elizabeth Warren, and also promised to stay quiet the Hong Kong protests as long as trade talks progressed, CNN reported on Thursday, citing two people familiar with the conversation.

A transcript of the call between them was stored in a highly secured electronic system, CNN said, which was also used to log a July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky.

During this call, Trump repeatedly pressed his Ukrainian counterpart to investigate Biden and his son ahead of the 2020 election. Biden is a frontrunner in the Democratic nomination for the presidency.

Read more: The White House 'lockdown' of Trump's call with Ukraine looks like part of a bigger, worrying pattern

The Financial Times had also reported in July that Trump promised to mute support for the anti-China movement in Hong Kong in exchange for re-opening trade talks. It's not clear if the two outlets were reporting on the same conversation.

US-China trade negotiations remain deadlocked, with the US reading a 30% tariff on $250 billion worth of Chinese goods later this month.

The phone call between Zelensky and Trump is at the center of a whistleblower complaint which alleges that Trump used the power of his office to "solicit interference from a foreign country" for personal gain.

The complaint has sparked a House's impeachment inquiry into Trump.

Warren responded to news of Trump's call with Xi on Thursday, tweeting that Trump was "sell[ing] out the people of Hong Kong behind closed doors." 

Details of Trump's call with Xi followed comments made by Trump on Thursday, in which he suggested that China should investigate Biden. 

"China should start an investigation into the Bidens because what happened in China is just about as bad as what happened with Ukraine," Trump told reporters in the White House lawn.

There has been no evidence of illegal activity by Biden or his son in Ukraine or China, despite Trump's claims. 

Chinese officials were baffled by Trump's comments. CNN said it was unclear if China would be launching an investigation, or if they would publicize such a move. 

"This is quite chaotic," a Chinese diplomat told CNN. "We do not want to get in the middle of the US politics."

Read more: Newly revealed text messages show Trump diplomats' internal turmoil over his pressure on Ukraine

Trump's record on Hong Kong

Trump has previously said the US was "carefully monitoring" the situation in Hong Kong but stopped short of supporting either China or Hong Kong.

In August, Trump called Xi a "good man" and said he had "ZERO doubt" that Xi could end protests "quickly and humanely."

On October 1, when China celebrated 70 years of Communist rule and Hong Kong protests descended into violence, Trump tweeted his congratulations to Beijing and made no mention of Hong Kong.

Read more: Barricades, human chains, and battling tear gas: We followed Hong Kong activists through the city's most turbulent protests yet

But Trump offered some sort of defense of the protests in a speech at the UN last month, in which he called on China to protect Hong Kong's freedoms. 

"How China chooses to handle this situation will say a great deal about its role in the world in the future," he said. "We are all counting on President Xi as a great leader."