Washington (CNN) President Donald Trump has been asking his advisers how unrest in Hong Kong is likely to unfold , according to senior administration officials — and some are warning that without a firmer US position there could be a bloodbath.

That includes national security adviser John Bolton and senior officials at the National Security Council. The White House and Trump have also heard from lawmakers, including those close to the administration like South Carolina Republican Sen. Lindsey Graham that inaction or caution in calling out China could end poorly.

Trump's response to Hong Kong's ongoing violence has remained largely muted, the officials said, in part because of concern about ongoing trade talks with Beijing. But with growing Republican frustration on Capitol Hill about his reticence, the President has shown flickering signs of adopting a firmer tone on China.

On Thursday morning, Trump once again appealed to President X Jinping's pride, tweeting that "if President Xi would meet directly and personally with the protesters, there would be a happy and enlightened ending to the Hong Kong problem. I have no doubt!"

'Big mistake'

The President's restraint in warning Beijing comes as China's ambassador to the UK warned Thursday that his government won't hesitate to intervene in Hong Kong, and Beijing maintains its build-up of troops on the border of the former British colony.

On Wednesday, Bolton issued a warning to China that it risks global wrath if its crackdown takes a more aggressive turn.

"The Chinese have to look very carefully at the steps they take because people in America remember Tiananmen Square , they remember the picture the man standing in front of the line of tanks," he told Voice of America in an interview, referencing the 1989 protests that Beijing brutally suppressed. "It would be a big mistake to create a new memory like that in Hong Kong."

The State Department also spoke up Wednesday, saying the "United States is deeply concerned by reports of Chinese paramilitary movement along the Hong Kong border," urging restraint and issuing a warning to Beijing about what it is risking.

The statement "strongly" urged Beijing to stick to its commitments in the Sino-British Joint Declaration to allow Hong Kong to exercise a high degree of autonomy and said the concerns of its citizens are legitimate. The State Department also rejected Beijing's attempts to paint the protests as US-driven or organized and included a warning to China that it could undermine Hong Kong's status as a global financial center.

"The ongoing demonstrations in Hong Kong reflect the sentiment of Hongkongers and their broad and legitimate concerns about the erosion of Hong Kong's autonomy," the statement said. "We categorically reject the false charge of foreign forces as the black hand behind the protests. The continued erosion of Hong Kong's autonomy puts at risk its long-established special status in international affairs."

Previously, administration officials had been advised by senior White House aides to adopt a more measured tone on Hong Kong, according to people familiar with the matter.

Trump, in early August, studiously wouldn't take sides when asked about the ongoing protests, which were sparked initially by plans to allow China to extradite people from Hong Kong and then, after that proposal was put on ice, broadened to encompass wider demands for democracy.

"Well something's probably happening with Hong Kong," Trump told reporters outside the White House on August 1. "But that's between Hong Kong and that's between China because Hong Kong is a part of China. They'll have to deal with that themselves. They don't need advice," he continued.

Ongoing US-China trade talks were one reason Trump wanted to stay largely quiet on Hong Kong, US officials said, alongside the fact the administration did not want to give more fuel to China's allegations that the US is to blame for the protests.

Critics in Congress

But the approach has drawn criticism from members of Congress, including House Speaker Nancy Pelosi, who have urged Trump to speak up in support of the pro-democracy protestors in Hong Kong in recent days.

Florida Republican Sen. Marco Rubio told CNN in an interview that Trump was likely concerned about endangering a potential trade deal, but that trade priorities should not override concerns about freedom abroad. Rubio added that he wished Trump had chosen different words when he said earlier this month that the protests were "riots" and that the issue was between Hong Kong and China.

"The president is a business man," said Rubio, who has been a vocal critic of the Chinese government. "As a business man, he's a very transactional person, so I think he views foreign policy in a very transactional way."

"My hope is that at some point we can adjust that thinking to realize that in fact those two issues cannot be separated, that he will have to confront, in any trade deal, how to treat products and services that come out of Hong Kong," Rubio said.

"We have to decide: do we want to be a defender of democracy? Do we want to be a defender of those principles that have made the world, in my view, safer? And do we think American democracy is safer with more democratic nations on earth," Rubio asked. "Or do we sort of want to just retreat within ourselves and believe that somehow we'll be better off with a world in which the principles that underlie who we are have eroded as a model for nations and for governance?"

If President Xi would meet directly and personally with the protesters, there would be a happy and enlightened ending to the Hong Kong problem. I have no doubt! https://t.co/eFxMjgsG1K — Donald J. Trump (@realDonaldTrump) August 15, 2019

Politico and the Financial Times have reported that Trump told Xi in a mid-June phone call that he would not interfere in the Hong Kong matter. CNN hasn't yet been able to confirm the phone call.

But in public remarks up until Wednesday, Trump had largely maintained that viewpoint. He told reporters on Tuesday he wanted the situation to work out for everyone, "including China," and did not condemn Beijing.

Commerce Secretary Wilbur Ross echoed that view during an interview on CNBC Wednesday morning: "I don't know if we would have done anything different in the past. What would we do? Invade Hong Kong?" he asked. "The president has made clear that he is watching very carefully what's happening. ...This is an internal matter."

Rubio condemned Ross' take, saying that, "it's not an internal matter. They're wrong if they say that," Rubio said, pointing to Hong Kong's unique international status and autonomy as special administrative region.

On Wednesday, Trump adopted a different tone, appealing to Xi through praise for his negotiating abilities and suggesting the two leaders meet.

'ZERO doubt'

"I know President Xi of China very well," he wrote on Twitter. "He is a great leader who very much has the respect of his people. He is also a good man in a 'tough business.' I have ZERO doubt that if President Xi wants to quickly and humanely solve the Hong Kong problem, he can do it. Personal meeting?"

He also publicly tied the trade talks to Hong Kong for the first time, acknowledging a link that administration officials have been making for months.

"Good things were stated on the call with China the other day. They are eating the Tariffs with the devaluation of their currency and "pouring" money into their system," he wrote. "The American consumer is fine with or without the September date, but much good will come from the short deferral to December."

"It actually helps China more than us, but will be reciprocated," Trump continued. "Millions of jobs are being lost in China to other non-Tariffed countries. Thousands of companies are leaving. Of course China wants to make a deal. Let them work humanely with Hong Kong first!"

Beijing, however, is sounding less amenable to working "humanely" with Hong Kong.

Chinese ambassador to the UK Liu Xiaoming warned that the government would not refrain from intervening in Hing Kong in a press conference Thursday.

A medical staffer helps a detained man, who protesters claimed was a police officer from mainland China, during a demonstration at the Airport in Hong Kong, Tuesday, Aug. 13, 2019. Protesters severely crippled operations at Hong Kong's international airport for a second day Tuesday, forcing authorities to cancel all remaining flights out of the city after demonstrators took over the terminals as part of their push for democratic reforms. (AP Photo/Kin Cheung)

Liu Xiaoming said that "should the situation deteriorate further into unrest ... the central government will not sit on its hands and watch. We have enough solutions and enough power ... to quell any unrest."

He added that the government was "fully prepared for the worst."

Trump claimed on Tuesday that US intelligence shows China was moving troops toward its border with Hong Kong. This comes after Chinese media published video of military police assembling in the city of Shenzhen, which borders Hong Kong, ahead of "large-scale exercises."

At least 6,000 Chinese troops are usually based in Hong Kong, according to a 2015 article published by the Ministry of National Defense of the People's Republic of China, though the Beijing government has never confirmed the exact number.

Both the PLA and the 1.5 million-strong People's Armed Police Force, whose members were seen in the recent video, report to the Central Military Commission headed by Xi.

US officials have been watching Beijing's troop movements very closely and so far, they do not see forces "streaming to the border" between China and Hong Kong, according a US official with direct knowledge of the latest assessment on Hong Kong. The US sense, for now, is that China is trying to signal its readiness to take action, but is still not ready to take that step.

'Sending a message'

The official noted, that the situation could quickly change as the forces are mobile and China has the capability to put significant forces into Hong Kong "within several hours."

The official says the forces on the Shenzen side are largely, if not all, People's Armed Police. In recent months, as the crisis has built up, the PAP has been at "increased readiness levels," and made public shows of force with anti-riot training.

The official said these displays are part of China "sending a message" to Hong Kong and the world that they are present and have the capability to move in if and when they like. PAP forces are staged in layers, some very close, some hours further back, the official said.

US officials believe China understands how economically disastrous it could be if their police move in, so they are trying to diffuse the situation without any interference, the official added.