SAN FRANCISCO • Facebook said on Monday that it was tightening its security for the 2020 United States elections, with stepped-up scrutiny of "state-controlled" media seeking to manipulate American voters.

The moves add to a series of measures from the leading social network since 2016, when foreign entities were prominently involved in social media in the US campaign. "The bottom line here is that elections have changed significantly since 2016, and Facebook has changed too," Facebook chief executive Mark Zuckerberg told a conference call. "We face increasingly sophisticated attacks from nation-states... but I'm confident we're more prepared now."

One of the new steps calls for the labelling of messages coming from state-controlled media outlets, starting next month. "We will hold these pages to a higher standard of transparency because they combine the opinion-making influence of a media organisation with the strategic backing of a state," Facebook said.

Facebook also said it would seek to curb the viral spread of misinformation by using a "pop-up" that will appear when people attempt to share posts on Instagram debunked by third-party fact-checkers. "In addition to clearer labels, we're also working to take faster action to prevent misinformation from going viral, especially given that quality reporting and fact-checking take time," it said. "In many countries, including in the US, if we have signals that a piece of content is false, we temporarily reduce its distribution pending review by a third-party fact-checker."

In a related development, Facebook said it removed four separate networks of accounts from Russia and Iran for "coordinated inauthentic behaviour" on Facebook and Instagram. Three of the groups originated in Iran and one in Russia, and they targeted users in the US, North Africa and Latin America.

One of the Russian groups used posts with concealed identities to make comments "on both sides of political issues, including topics such as US elections, environmental issues, racial tensions, LGBTQ issues, political candidates, confederate ideas, conservatism and liberalism," Facebook said.

An Iranian account which "masqueraded as a news entity" posted on topics including race relations, US and Israeli policies on Iran and the Black Lives Matter movement.