In a tweet Mr Trump said he had “ZERO doubt” that Mr Xi could “humanely solve the Hong Kong problem”.
He also tied the protests to a US trade deal with Beijing, in the face of ongoing trade tensions.
“Of course China wants to make a deal. Let them work humanely with Hong Kong first!” he tweeted.
Mr Trump’s comments come after weeks of tumultuous pro-democracy protests sparked by opposition to an extradition bill in Hong Kong.
Critics feared the bill would erode freedoms in Hong Kong, by allowing suspects to be extradited to mainland China.
The bill has now been suspended, but the protests have evolved into a broader movement, with demonstrators expressing anger at alleged police brutality, and demanding democratic reform.
Hong Kong, a former British colony, is part of China under a “one country, two systems” model that grants it a high level of autonomy.
It has its own legal system and judiciary and enjoys certain freedoms not seen in the mainland – Hong Kong and Macau for example, are the only places in Chinese territory where people can hold vigils commemorating the 1989 Tiananmen Square crackdown.
Tensions and tear gas
Clashes between demonstrators and police have escalated in recent days, with Hong Kong leader Carrie Lam warning the city could be “pushed into an abyss”.
The Chinese government has strongly criticised the protesters, calling their behaviour “close to terrorism”.
On Wednesday evening, police armed with riot shields fired tear gas to disperse hundreds of demonstrators gathered in the Sham Shui Po neighbourhood, who had been shining laser pointers at a police station.
Laser pointers have gained significance in the protests after a student was arrested for possessing laser pointers, which police described as an “offensive weapon” that could cause serious eye injuries.
Since then, demonstrators have used lasers as a way of ridiculing the claim.
Wednesday’s tear gas came a day after protests at Hong Kong’s International Airport turned violent on Tuesday night.
Thousands of protesters flooded the terminal buildings, using luggage trolleys to build barriers.
The gathering started off as a peaceful mass sit-down, but things escalated and a mainland Chinese state media journalist was set upon by protesters who accused him of being an undercover police officer.
The situation deteriorated further after police officers used pepper spray against some demonstrators while trying to evacuate an injured man.
In one instance, a policeman was seen frantically drawing his gun after being attacked with his own truncheon for manhandling a woman.
Hong Kong police said the officer’s life was “under great danger” and insisted he only drew his gun “out of emergency and necessity”.
Relations between protesters and police have already been tense, but deteriorated further in recent days, after police were seen firing pepper ball rounds on protesters at close range, and firing tear gas in an enclosed train station, during protests on Sunday.
Amid continued violent clashes, and strengthening rhetoric from Beijing, some observers have expressed fears of a direct military intervention by China – though most analysts believe this is unlikely at this stage.
Separately, the US president’s national security advisor, John Bolton, warned China on Wednesday to tread “carefully” in Hong Kong “because people in America remember Tiananmen Square”.
A repeat of the military crackdown on the 1989 student-led protests in China would be a “big mistake”, he told news outlet Voice of America.
China’s ambassador to the UK, Liu Xiaoming, said on Thursday that Hong Kong was at a “critical moment”.
“Should the situation in Hong Kong deteriorate further… the central government will not sit on its hands and watch,” he said at a press conference in London.
“We have enough solutions and enough power within the limits of (the) Basic Law to quell any unrest swiftly,” Mr Liu added.
While China’s People’s Liberation Army (PLA) has troops stationed in Hong Kong, they are not expected to interfere in local issues.
However, Hong Kong’s mini constitution, known as the Basic Law, does allow the government to request the PLA’s assistance for maintaining public order, or disaster relief.
So far, Hong Kong’s government and police have said they have no plans to involve the army.