President Donald Trump promised Chinese President Xi Jinping in a June phone call that he would stay quiet on the protests in Hong Kong in exchange for progress in the trade war, CNN reported Thursday.

Trump has so far offered a muted response to the monthslong protests, despite global backlash over China's encroachment on the semi-autonomous city.

Trump earlier Thursday suggested that China should investigate former Vice President Joe Biden.

Another of Trump's phone calls with foreign leaders is at the center of a whistleblower complaint that 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 protests that have rocked Hong Kong since June, and his trade war with China might be the reason.

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

Former Vice President Joe Biden and Sen. Elizabeth Warren, the two 2020 Democratic presidential candidates Trump reportedly brought up with Xi in a phone call. Mike Blake/Reuters

CNN said the transcript of that call was stored in the highly secured electronic system also used for the transcript of the July 25 phone call between Trump and Ukrainian President Volodymyr Zelensky during which Trump repeatedly pressed his Ukrainian counterpart to investigate Biden — a frontrunner for the Democratic presidential nomination — and his son ahead of the 2020 election.

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

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

US-China trade negotiations remain deadlocked, and the US is set to impose a 30% tariff on $250 billion worth of Chinese goods later this month.

Protesters walk through tear-gas smoke in Hong Kong on August 4. Vernon Yuen/NurPhoto via Getty Images

Trump's call with Zelensky is at the center of a whistleblower complaint alleging that Trump used the power of his office "to solicit interference from a foreign country" for political gain. The complaint sparked House Democrats' impeachment inquiry into Trump.

Warren responded to the news of Trump's call with Xi with a Thursday tweet saying that it was "outrageous that any president would sell out the people of Hong Kong behind closed doors."

Earlier Thursday, Trump 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 on the White House lawn.

There is no evidence of illegal activity by Biden or his son in Ukraine or China.

Chinese officials were baffled by Trump's comments, CNN said, adding that it was unclear whether China would launch an investigation or 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 on the phone with Saudi King Salman in the White House in January 2017. Mark Wilson/Getty Images

Trump's record on Hong Kong

Trump has said the US is "carefully monitoring" the situation in Hong Kong but has 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 Tuesday, when China celebrated 70 years of Communist rule and the Hong Kong protests descended into violence, Trump tweeted to congratulate Beijing, making 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

Protesters in gas masks in Hong Kong. Anthony Kwan/Getty Images

But Trump offered some sort of defense of the protests in a speech at the UN last month, calling 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."