A man pleaded guilty on Thursday to assaulting US Representative Ilhan Omar during a January town hall in which the Democratic lawmaker condemned Republican US President Donald Trump's immigration crackdown, the US Justice Department said.

"Anthony James Kazmierczak, 55, pleaded guilty today to one count of Assaulting a United States Officer in US District Court," the Justice Department said in a statement.

Kazmierczak admitted in a Thursday hearing before US District Judge Joan Ericksen that he "planned the assault and that he did not agree with Representative Omar's political views," the Justice Department said.

In her remarks during the town hall, Omar was criticizing the US Immigration and Customs Enforcement agency for its crackdown in Minnesota and the fatal shootings by federal agents of two US citizens in Minneapolis.

Trump's crackdown and ICE detentions have been widely condemned by human rights groups who call them a violation of due process and free speech rights. Rights advocates also say the crackdown has created an unsafe environment, particularly for ethnic minorities.

Kazmierczak sprayed Omar with apple cider vinegar from a syringe, which landed on Omar's clothing and skin, the Justice Department said, adding that lab analysis confirmed the liquid contained acetic acid. Omar was uninjured in the attack.

Omar, who is Muslim, came to the US as a 12-year-old girl and became an American citizen in 2000.

Trump has repeatedly targeted Omar in public remarks and social media posts, while also taking aim at her Somali nationality and saying she should be "institutionalized" and removed from the US.

Kazmierczak gestured and shouted at Omar during the town hall before security guards subdued him. He was subsequently arrested. The town hall meeting in Minneapolis was temporarily disrupted, but Omar later continued her remarks.

In recent years, political experts have raised an alarm about political violence in a polarized US.