In this file photo, US President Donald Trump speaks about the coronavirus in the James Brady Press Briefing Room of the White House, April 22, 2020, in Washington. (ALEX BRANDON / AP)

SAN FRANCISCO - President Donald Trump threatened to regulate or shutter social media companies -- a warning apparently aimed at Twitter Inc after it began fact-checking his tweets.

In a pair of tweets issued Wednesday morning from his iPhone, Trump said that social media sites are trying to silence conservative voices

In a pair of tweets issued Wednesday morning from his iPhone, Trump said that social media sites are trying to silence conservative voices, and need to change course or face action.

There is no evidence that Trump has the ability to shut down social media networks, which are run by publicly traded companies and used by billions of people all over the world.

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“Republicans feel that Social Media Platforms totally silence conservatives voices. We will strongly regulate, or close them down, before we can ever allow this to happen,” he said Wednesday. In a second tweet, he added: “Just like we can’t let large scale Mail-In Ballots take root in our Country.”

He didn’t cite any platforms by name, but it was plainly a response after Twitter added a fact-check label to earlier Trump tweets that made unsubstantiated claims about mail-in voting.

Twitter shares fell 1.65 percent in early trading in New York after Trump’s tweet. Twitter didn’t immediately respond to a request for comment Wednesday morning.

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Twitter on Tuesday prompted readers to check the facts in tweets sent by Trump, warning that his claims about mail-in ballots were false and had been debunked by fact checkers.

The move marked a dramatic shift for the social network, Trump’s primary tool for getting an unfiltered version of his message out to his political base, after years of permissive policies around content on its platform.

The company has been tightening those policies in recent years amid criticism that its hands-off approach had allowed abuse, fake accounts and misinformation to thrive.

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Trump, who has more than 80 million followers on Twitter, claimed in tweets earlier in the day that mail-in ballots would be “substantially fraudulent” and result in a “rigged election.” He also singled out the governor of California over the issue, although the state is not the only one to use mail-in ballots.

Hours later, Twitter posted a blue exclamation mark alert underneath those tweets, prompting readers to “get the facts about mail-in ballots” and directing them to a page with information aggregated by Twitter staffers about the claims.

A headline at the top of the page stated “Trump makes unsubstantiated claim that mail-in ballots will lead to voter fraud,” and was followed by a “what you need to know” section addressing three specific claims made in the tweets.

Twitter said the application of a fact-checking label to the president’s tweets was an extension of its new “misleading information” policy

Social media companies such as Twitter, Facebook Inc and Google’s YouTube have been swept up in recent years in a broader backlash against tech in Washington, state capitals and even Europe.

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Trump has little authority to shut the businesses, which are also central to his re-election campaign and communications strategy with the public. But they already face a range of potential regulations.

White House adviser Kellyanne Conway, speaking to reporters at the White House on Wednesday, side-stepped questions of what measures in particular Trump may take. She said social media helps conservatives skirt the mainstream news media.

“The president’s saying please stop suppressing conservative voices,” she said, before criticizing Twitter’s fact check. “I thought using outlets that are decisively and proudly anti-Trump to fact-check the president was maybe the richest piece of the whole thing.”

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Twitter had said on Tuesday that the application of a fact-checking label to the president’s tweets was an extension of its new “misleading information” policy, introduced earlier this month to combat misinformation about the coronavirus.

It said at the time that it would later extend the COVID-19 policy to other types of disputed or misleading information.

Twitter so far has used its policies sparingly against major political figures, but did delete tweets by the presidents of Brazil and Venezuela which violated its coronavirus rules.

The company’s alert on Trump’s mail-in ballot tweets came hours after it declined to take action on separate tweets Trump had sent about the 2001 death of a former congressional staff member for Joe Scarborough, after her widower asked the company to remove them for furthering false claims.

A Twitter spokesman told Reuters Trump’s mail-in ballot tweets were related to election integrity and therefore subject to different treatment under its policies.