A US veteran of the war in Afghanistan who prosecutors say plotted to detonate a nail bomb at a Los Angeles-area white nationalist rally, hoping to cause mass casualties, has been arrested in an FBI sting.

Mark Domingo told an FBI informant in a private chat room in March that he wanted to carry out an attack against America in retribution for the Christchurch mosque massacres that killed 50 people, according to prosecutors.

Domingo, 26, a US Army infantryman who fought in Afghanistan and recently converted to Islam, was taken into custody on Friday after being given what he thought was a live bomb by the informant, officials said.

“Often we are asked what keeps us up at night. This is a case that keeps us up at night,” Ryan Young, special agent in charge of the FBI’s Joint Terrorism Task Force, told a news conference in Los Angeles.

“This investigation successfully disrupted a very real threat posed by a trained combat soldier who repeatedly stated he wanted to cause the maximum number of casualties.”

Domingo, who had purchased several hundred long nails to serve as shrapnel in an improvised explosive device, had also allegedly suggested attacks on Jews, police officers, churches, a military facility, Southern California freeways and the Santa Monica Pier during internet conversations with the informant.

He was charged in a federal criminal complaint with providing material support to terrorists and was ordered held without bond during a brief initial appearance in US District Court in Los Angeles on Monday.

During the initial conversations with the informant, Domingo allegedly suggested “America needs another Vegas event”, an apparent reference to the 2017 mass shooting at a country music concert in Las Vegas, where a lone gunman perched in a nearby hotel killed 58 people and wounded hundreds.

Domingo, who said he wanted to become a martyr in a “violent jihad,” also referred to the 2013 Boston Marathon bombing, according to the complaint. During one meeting in person, he arrived carrying an AK 47-style assault rifle.

Prosecutors say Domingo contemplated drive-by shootings and detonating a bomb at the Santa Monica pier. He eventually decided to detonate an improvised explosive device at an April 28, 2019, rally in the Los Angeles suburb of Long Beach that he believed was organised by white nationalists.

-with AAP