White supremacists have the "most persistent and concerning transnational connections" of any violent domestic extremist group in the US, according to an unclassified summary of a joint threat assessment released by the Office of the Director of National Intelligence on Wednesday.

The assessment said this is because individuals with similar ideological beliefs exist outside of the US, and such groups "frequently communicate with and seek to influence each other."

"We assess that a small number of US [racially or ethnically motivated violent extremists] have traveled abroad to network with like-minded individuals," the report added.

The assessment of the national security threat posed by domestic violent extremism was ordered by the White House in January and produced by the ODNI as well as the Departments of Justice and Homeland Security.

The transnational links between white supremacist groups, fueled in part by social media, have been a growing concern for US officials and extremism watchdogs in recent years.

"The danger of terrorism is growing in the United States, just as it is elsewhere in the world, with white supremacist extremists strengthening transnational networks and even imitating the tactics, techniques, and procedures of groups like al-Qaeda and the Islamic State," a 2019 report from the Soufan Center states.

Law enforcement in the US and the intelligence community have been ringing alarm bells on the threat of domestic extremism, particularly as it pertains to the far right, for years. The Capitol attack on January 6 pushed this topic to the forefront of the nation's attention, and it's increasingly at the center of conversations surrounding national security.

The assessment released by the ODNI on Wednesday said racially motivated extremists and militia extremists pose the most lethal domestic terror threat, while warning domestic violent extremism presents an "elevated threat" to the homeland in 2021.

Homeland Security chief Alejandro Mayorkas on Wednesday told congressional lawmakers domestic violent extremism is the "greatest threat" to the US.

"Right now, at this point in time, domestic violent extremism, the lone wolf, the loose affiliation of individuals following ideologies of hate and other ideologies of extremism that are willing and able to take those ideologies and execute on them in unlawful, illegal, violent ways is our greatest threat in the homeland right now," Mayorkas said.