WASHINGTON: The US Justice Department is considering legal changes to combat what it sees as a rising threat from anti-government domestic extremists, senior officials told Reuters, even as it steps up efforts to stop Islamic State-inspired attacks at home.

Extremist groups motivated by a range of US-born philosophies present a "clear and present danger," John Carlin, the Justice Department's chief of national security, told Reuters in an interview. "Based on recent reports and the cases we are seeing, it seems like we're in a heightened environment."

Over the past year, the Justice Department has brought charges against domestic extremist suspects accused of attempting to bomb US military bases, kill police officers and fire bomb a school and other buildings in a predominantly Muslim town in New York state.

But federal prosecutors tackling domestic extremists still lack an important legal tool they have used extensively in dozens of prosecutions against Islamic State-inspired suspects: a law that prohibits supporting designated terrorist groups.

Carlin and other Justice Department officials declined to say if they would ask Congress for a comparable domestic extremist statute, or comment on what other changes they might pursue to toughen the fight against anti-government domestic extremists.

The US State Department designates international terrorist organisations to which it is illegal to provide "material support." No domestic groups have that designation, helping to create a disparity in charges faced by international extremist suspects compared to domestic ones.

A Reuters analysis of more than 100 federal cases found that domestic terrorism suspects collectively have faced less severe charges than those accused of acting on behalf of Islamic State since prosecutors began targeting that group in early 2014.

Graphic: http://tmsnrt.rs/1IbZGHR

Over the past two years, 27 defendants have been charged with plotting or inciting attacks within the United States in the name of Islamic State. They have faced charges that carried a median prison sentence of 53 years - half of the defendants faced more, and half faced less.

In the same period, 27 adherents of US-based anti-government ideologies have been charged with similar activity. They faced charges that carried a median prison sentence of 20 years.

Carlin said his counter-terrorism team, including a recently hired counsel, is taking a "thoughtful look at the nature and scope of the domestic terrorism threat" and helping to analyse "potential legal improvements and enhancements to better combat those threats."

The counsel, who was appointed last October and has not been named publicly, will identify cases being prosecuted at the state level that "could arguably meet the federal definition of domestic terrorism," a Justice Department official said.

That would give the department a direct role in more domestic extremism cases.

Recognising that domestic threats were "rapidly evolving, and had the potential to grow," the department in March 2015 rated disrupting such terrorists as a key component of its broader counter-terrorism efforts, officials said.