WASHINGTON (BLOOMBERG) - US President Donald Trump's campaign against what he calls unfair media coverage has a new target: Google and other online companies that increasingly deliver the news to the public.

In a series of tweets and public comments on Tuesday (Aug 28), Mr Trump warned Alphabet Inc's Google, Facebook Inc and Twitter Inc that they "better be careful", though he didn't specify what actions he would - or could - take against them.

Mr Trump claimed without providing evidence that Google's news search function favoured liberal over conservative outlets.

Later, in a meeting in the Oval Office, he told reporters that the three technology companies "are treading on very, very troubled territory".

Mr Trump's re-election campaign also texted the President's tweets to supporters, writing as part of an end-of-month fund-raising push that "The FAKE NEWS machine is completely out of control".

Experts say it's unclear what he or Congress could do to influence how Internet companies distribute online news. The industry has an exemption from liability for the content users post.

Some top members of Congress have suggested limiting that protection as a response to allegations of bias and other misdeeds, although there have been few moves to do so since Congress curbed the shield for some cases of sex trafficking earlier in the year.

The government has little ability to dictate to publishers and online curators what news to present despite the President's occasional threats to use the power of the government to curb coverage he dislikes and his tendency to complain that news about him is overly negative.

Mr Trump has talked about expanding libel laws and mused about reinstating long-ended rules requiring broadcasters to offer equal time for opposing views, which didn't apply to the Internet. Neither has resulted in a serious policy push.

"From a standpoint of regulating content, there's immediately a gigantic First Amendment problem for any effort," said Mr Larry Downes, project director at the Georgetown Centre for Business and Public Policy.

Mr Downes said that it's possible the Federal Trade Commission could use its authority over deceptive practices to examine the company's statements that it doesn't skew its results based on politics.

Such an inquiry, though, would require a "charitable" evaluation of the government's powers, he said, noting that Mr Trump is not supposed to interfere in the independent agency.

The accusation of bias, dismissed by online search experts, follows the President's Aug 24 statement that social media "giants" are "silencing millions of people".

Such allegations - along with assertions that the news media and Special Counsel Robert Mueller's investigation are arrayed against him - have been a chief Trump talking point meant to appeal to his base.

Google issued a statement saying its searches are designed to give users relevant answers.

"Search is not used to set a political agenda and we don't bias our results towards any political ideology," the statement said.

Twitter has said that any allegations of anti-conservative bias are unfounded.

Facebook didn't respond to a request for comment on Mr Trump's allegations.

The threats drew a strong rebuke from free-speech advocates.

"There are lots of things that get published that political candidates, businessmen and people on the street don't like," said Dr Lucy Dalglish, dean of the journalism school at the University of Maryland.

"But you don't get to ban speech because you don't like what was said or published."

Freedom Partners, a conservative group affiliated with billionaire Charles Koch that has taken issue with some of Mr Trump's policies, called any attempt to regulate search engines "a dangerous precedent".

"Regulating the results of Google or other tech companies is a reckless idea that would undermine essential elements of free speech," Mr Jim Fellinger, a spokesman, said in a statement.

Google's rankings have long angered media companies, not for political bias but for the revenue that the search company gets from news publishers' work and concerns about the promotion of controversial outlets at the expense of those doing careful reporting and breaking original stories.

Mr Rupert Murdoch, who controls, among other news organisations, the Wall Street Journal, has called on Facebook to pay publishers fees to carry the news that its users post and share online.

Lately, conservatives have taken up the cause.

Google News rankings have sometimes highlighted unconfirmed and erroneous reports in the early minutes of tragedies when there's little information to fill its search results.

After the Oct 1, 2017, Las Vegas shooting, for instance, several accounts seemed to coordinate an effort to smear a man misidentified as the killer with false claims about his political ties.

Google has since tightened requirements for inclusion in news rankings, blocking outlets that "conceal their country of origin" and relying more on authoritative sources, although the moves have led to charges of censorship from less established outlets.

Google currently says it ranks news based on "freshness" and "diversity" of the stories. Trump-favoured outlets such as Fox News routinely appear in results.

But not enough for the President.

"Google search results for 'Trump News' shows only the viewing/reporting of Fake New Media. In other words, they have it RIGGED, for me & others, so that almost all stories & news is BAD," Mr Trump tweeted on Monday morning. "Republican/Conservative & Fair Media is shut out. Illegal."

Mr Trump's comment followed a Monday evening report on Fox Business that said 96 per cent of Google News results for "Trump" came from the "national left-wing media".

The segment cited the conservative PJ Media site, which said its analysis suggested "a pattern of bias against right-leaning content".

The PJ Media analysis "is in no way scientific", said Mr Joshua New, a senior policy analyst with the Centre for Data Innovation.

"This frequency of appearance in an arbitrary search at one time is in no way indicating a bias or a slant," Mr New said.

His non-partisan policy group is affiliated with the Information Technology and Innovation Foundation, which in turn has executives from Silicon Valley companies, including Google, on its board of directors.

Twitter has addressed concerns over "shadow banning", or deliberately making someone's content undiscovered to everyone except the person who posted it, writing in a blog post last month that Twitter does not use those tactics.

"Our purpose is to serve the conversation, not to make value judgement on personal beliefs," Mr Nick Pickles, Twitter's global lead for public policy strategy, said in last month's congressional hearing.

Google said it issues hundreds of improvements every year to its algorithms. "We continually work to improve Google Search and we never rank search results to manipulate political sentiment," the company said in its statement.

Ever since Facebook faced its first allegations of anti-conservative bias in 2016, because of employees who decided what stories were considered "trending", the company has tried to remove itself from making direct decisions about content.

Outside academic reviews of Facebook have found no evidence of anti-conservative bias.

But the company has enlisted former Republican Senator Jon Kyl of Arizona to investigate the issue internally.

Google's search results have been the focus of complaints for more than a decade. The criticism has become more political as the power and reach of online services has increased in recent years.

Mr Eric Schmidt, Alphabet's former chairman, supported Mrs Hillary Clinton in 2016. There have been unsubstantiated claims the company buried negative search results about her during the campaign. Scores of Google employees entered government to work under former president Barack Obama.

But Google's news search software doesn't work the way the President says it does, according to Mr Mark Irvine, senior data scientist at WordStream, a company that helps firms get online content to show up higher in search results.

The Google News system gives weight to how many times a story has been linked to, as well as to how prominently the terms people are searching for show up in the stories, Mr Irvine said.

"The Google search algorithm is a fairly agnostic and apathetic algorithm towards what people's political feelings are," he said.

Google is to face lawmakers at a hearing on Russian election meddling on Sept 5.

The company intended to send Senior Vice-President for Global Affairs Kent Walker to testify, but the panel's chairman, Senator Richard Burr of North Carolina, who wanted Chief Executive Officer Sundar Pichai, has rejected Mr Walker.

Mr Trump has not always been hostile to Google. In July, he lashed out at the European Union for imposing a record US$5 billion (S$6.8 billion) fine against the company over its mobile phone operating system, calling Google one of America's "great companies".