US security officials have classified the left-wing group Antifa as "domestic terrorists", confidential documents have revealed.

The Department of Homeland Security (DHS) has reportedly been warning about the growing threat of violence between left-wing anarchists and right-wing nationalists since 2016, amid claims Antifa's activity has become more confrontational, according to documents seen by Politico.

A confidential intelligence report by the DHS and the FBI accused the "anarchist extremists" of attacks on police, government and political institutions, along with any other symbols of the "capitalist system" or displays of racism, social injustice or fascism.

It described some of their activities as "domestic terrorist violence".

Antifa, shorthand for anti-fascist organisations, refers to a loose coalition of decentralised, grassroots groups opposed to the many guises of the extreme right.

The group has come under growing scrutiny in the past weeks after Donald Trump blamed the "alt-left" for the deadly violence between white supremacists and counter protesters in Charlottesville, Virginia.

But law enforcement officials said that the US President helped spur the backlash through his own divisive rhetoric.

“It was in that period [as the Trump campaign emerged] that we really became aware of them,” one senior law enforcement official said.

“These antifa guys were showing up with weapons, shields and bike helmets and just beating the shit out of people ... they’re using Molotov cocktails, they’re starting fires, they’re throwing bombs and smashing windows.”

Violence on the streets of Charlottesville Show all 9 1 / 9 Violence on the streets of Charlottesville Violence on the streets of Charlottesville Protesters clash and several are injured White nationalist demonstrators clash with counter demonstrators at the entrance to Lee Park in Charlottesville, Virginia. A state of emergency is declared, August 12 2017 Violence on the streets of Charlottesville Trump supporters at the protest A white nationalist demonstrator walks into Lee Park in Charlottesville, Va., Saturday, Aug. 12, 2017. Hundreds of people chanted, threw punches, hurled water bottles and unleashed chemical sprays on each other Saturday after violence erupted at a white nationalist rally in Virginia. AP Photo Violence on the streets of Charlottesville State police stand ready in riot gear Virginia State Police cordon off an area around the site where a car ran into a group of protesters after a white nationalist rally in Charlottesville, Virginia AP Photo Violence on the streets of Charlottesville Militia armed with assault rifles White nationalists, neo-Nazis and members of the 'alt-right' with body armor and combat weapons evacuate comrades who were pepper sprayed after the 'Unite the Right' rally was declared a unlawful gathering by Virginia State Police. Militia members marched through the city earlier in the day, armed with assault rifles. Getty Images Violence on the streets of Charlottesville Statue of Confederate General Robert E Lee The statue of Confederate General Robert E. Lee stands behind a crowd of hundreds of white nationalists, neo-Nazis and members of the 'alt-right' during the 'Unite the Right' rally 12 August 2017 in Charlottesville, Virginia. They are protesting the removal of the statue from Emancipation Park in the city. Getty Images Violence on the streets of Charlottesville Racial tensions sparked the violence White nationalists, neo-Nazis and members of the 'alt-right' exchange insults with counter-protesters as they attempt to guard the entrance to Lee Park during the 'Unite the Right' rally Getty Violence on the streets of Charlottesville A car plows through protesters A vehicle drives into a group of protesters demonstrating against a white nationalist rally in Charlottesville, Virginia. The incident resulted in multiple injuries, some life-threatening, and one death. AP Photo Violence on the streets of Charlottesville Rescue personnel help injured people after a car ran into a large group of protesters after an white nationalist rally in Charlottesville, Virginia AP Photo Violence on the streets of Charlottesville President Donald Trump speaks about the ongoing situation in Charlottesville, Virginia from his golf club in Bedminster, New Jersey. He spoke about "loyalty" and "healing wounds" left by decades of racism.

Officials are now warning of an escalation of violence as white supremacist and far-right groups grow stronger and more militant.

“Everybody is wondering, 'What are we gonna do? How are we gonna deal with this?'” said the senior state law enforcement official.

“Every time they have one of these protests where both sides are bringing guns, there are sphincters tightening in my world. Emotions get high, and fingers get twitchy on the trigger.”

In their April 2016 assessment, the DHS and FBI said the Antifa could become more dangerous if “fascist, nationalist, racist or anti-immigrant parties obtain greater prominence or local political power in the United States, leading to anti-racist violent backlash from anarchist extremists”.

The comes as close to 350,000 people signed a petition calling on Mr Trump to “formally recognise” anti-fascists as terrorists.

The petition, which was created following the Virginia violence, urged the federal government to declare Antifa a terror group out of “principle, integrity, morality and safety”.