Homeland Security and the FBI have deemed antifa 'domestic terrorists' as they warn of escalating violence between the anti-fascist group and neo-Nazis.

Federal officials launched a global investigation into antifa last year to determine whether they may start committing terror attacks, like their right-wing counterparts such as the recent murder of Heather D. Heyer, 32, by a white supremacists who drove his car into a crowd of protesters in Charlottesville.

They also feared that antifa members, who have been seen at counter protests to white supremacist rallies, could travel abroad train with 'foreign anarchist extremist movements' in Greece, Italy and Mexico. Officials fear that as the nationalist movements grow under President Trump, the antifa movement will grow in response.

Homeland Security and the FBI have deemed antifa 'domestic terrorists' as they warn of escalating violence between the anti-fascist group and neo-Nazis (Antifa members and counter protesters gather during a rightwing No-To-Marxism rally on August 27, 2017 at Martin Luther King Jr. Park in Berkeley, California)

Federal officials launched a global investigation into antifa last year (Antifa members at a counter protest to the No-To-Marxism rally on August 27)

Even before the recent violent clash in Charlottesville, the Department of Homeland Security had warned that about the growing risk of violence between the left-wing antifa and right-wing groups.

They even formally classified their activities as 'domestic terrorist violence,' law enforcement sourced told Politico.

A senior state law enforcement official told them that 'a whole bunch of them' have been placed on U.S. terrorism watch lists.

Antifa, short for anti-fascist or Anti Fascist Action, has been around since the 1920s where it emerged as a response to the growing Nazism spreading across Europe.

The antifa protesters held a counter demonstration against the Unite the Right rally in Charlottesville

Since then, they have existed mainly on the fringes, but have gathered in force over the years to oppose racist, violent fascist groups, from clashes with Nazis on the streets of Germany, Italy, and Spain in the 1930s, to taking on the skinheads of the 1980s.

Now as America's extremist right wing groups become emboldened and more vocal under President Trump, the anti-fascist movement have had a huge boost in popularity as people join the ranks of the left wing group to oppose the alt-right.

An official said anti-fascist protesters began turning up at Trump rallies, confederate events and nationalist marches.

'These antifa guys were showing up with weapons, shields and bike helmets and just beating the s**t out of people,' the official said. 'They're using Molotov cocktails, they're starting fires, they're throwing bombs and smashing windows.'

Authorities blamed the left-wing group for instigating the violence, and say that as they clashed with the right-wing protesters and Trump supporters, they responded with their own weapons and violence.

Riot police stand before Antifa members and counter protesters during a right-wing No To Marxism rally on August 27 in Berkeley

Violence broke out during protests in Berkeley, California, and Sacramento as well as several clashes in Charlottesville before the most recent deadly rally.

The DHS report claimed that at the Sacramento rally, antifa protesters came prepared for violence, and revealed they were highly organized, with 'proficiency in pre-operational planning... raising bail money in preparation for arrests, counter-surveilling law enforcement using three-man scout teams, using handheld radios for communication, and coordinating the event via social media.'

'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.'

Once seen as rather extreme left wing faction, the antifa are generally anarchists who believe the best way to take on the alt-right is through direct action.

'We are... in the fightin' Nazis business,' said Antifa member and The Nation journalist Natasha Lennard, in a piece titled 'It's time to make Nazis afraid again.

'Antifa is a promise to neo-Nazis and their bedfellows that we will confront them in the streets; we will expose them online and inform their place of employ. We are not venues to deny space to far right events; we are vowing that all far-right events will be bombarded and besieged.'

Antifa-aligned journal It's Going Down warns that members 'have to organize to defend ourselves' against the alt-right.

Members of Antifa cheer as white activists are evicted from Emancipation Park during the Unite the Right Rally in Charlottesville, August 12

'If we allow the alt-right and neo-Nazis to organize in our communities, the consequences will be fatal,' they wrote. 'Charlottesville is just the beginning.'

'We have to stand up to white supremacists, we have to shut down and chase out these bigots every time they try to organize, or else they will kill more people. The police will not protect us.'

Portland, Oregon, has seen some of the worst clashes as the city, a mainstay of liberal, antifa activists, went up against neo-Nazis in a state which once had the highest Ku Klux Klan membership rate in America.

Following Trump's surprise election, masked protesters held demonstrations which saw store windows smashed and vandalism across the city. Violence erupted at a Rally for Trump and Freedom in the suburb of Vancouver after activists threw smoke bombs.

They have clashed again and again since then, including at a June rally, which featured alt-right's Kyle Chapman, who shot to internet fame for attacking an antifa activist with a wooden post. Antifa responded by throwing bricks until the police dispersed them.

Federal authorities told Politico that extremists on both sides have been using the violence to recruit for new members and antifa has established groups in numerous cities.

Despite their rise to prominence, authorities have struggled to penetrate antifa deeply enough to fully understand its capacity and its potential for violence.

But as the alt-right continues to flourish under Trump, the threat of antifa will only grow to meet it, meaning bigger and bloodier clashes.