WASHINGTON (Reuters) - Extremist groups proliferated in the United States in 2015, a U.S. research group said on Wednesday, as ideologically driven violence and incendiary political rhetoric created fertile conditions for white supremacists and other fringe actors.

The Southern Poverty Law Center said the number of far-right "hate groups" grew to 892 in 2015 from 784 the year before, spurred by battles over the Confederate flag, gay marriage, immigration and Islamic terrorism.

The SPLC also reported an increase in armed citizen militias, to 998 in 2015 from 874 the year before, as antigovernment activists were energized by land-use clashes with the federal government in the West and fears the Obama administration may tighten gun laws.

On the other end of the political spectrum, black separatist groups advocating anti-Semitic views grew to 180 chapters last year, up from 113 in 2014, according to the SPLC. Unlike activists in the Black Lives Matter movement who have sought to reduce police violence, black separatist groups like Israel United in Christ have advocated anti-Semitic and anti-white views.

Not all extremist movements have flourished. Established neo-Nazi groups like the National Alliance and the Aryans Nations have been hobbled by financial woes and leadership battles, the SPLC said.

But the number of Ku Klux Klan chapters more than doubled to 190 from 72, the SPLC said, invigorated by more than 300 rallies after the state of South Carolina took down the Confederate battle flag from its capitol grounds in the wake of a June massacre of nine African-American churchgoers in Charleston.

Stormfront, a white supremacist Web site, has been adding about 25,000 registered members annually for the past several years and now counts more than 300,000 members, according to the SPLC.

White supremacists have told Reuters that they are encouraged by the success of Republican presidential candidate Donald Trump, who has referred to Mexicans as "rapists" and called for a ban on all Muslim immigrants.

The December massacre of 14 people in San Bernardino, California by two Islamic State supporters spurred a wave of vandalism and harassment directed at U.S. Muslims.

According to several estimates, 2015 saw a new peak in domestic political violence. The Anti-Defamation League estimated that at least 52 people were killed by domestic extremists, the highest figure since the 1995 Oklahoma City bombing, which killed 168 people.

The increase in extremist activity has drawn the attention of the U.S. Justice Department, which is stepping up efforts to head off violent attacks in far-right groups.

Reporting by Andy Sullivan.

To Learn More:

Trump Anti-Immigrant Rhetoric Emboldens Hate Mongers (by Noel Brinkerhoff, AllGov)

Hate Groups Top 1,000 for First Time (by Noel Brinkerhoff, AllGov)