Several people were injured Sunday in what the FBI called a "targeted terror attack" in the US state of Colorado, where a group was protesting for the release of Israeli hostages held in Gaza.

"We are aware of and fully investigating a targeted terror attack in Boulder, Colorado," FBI chief Kash Patel said on X.

Local media cited eyewitnesses describing a man throwing something resembling a homemade Molotov cocktail at a group calling for the release of Israeli hostages held in Gaza. 

Asked if it was a terror attack against the protesters, Boulder Police Chief Steve Redfearn insisted it was "way too early to speculate motive" behind the incident, which took place shortly before 1:30 pm (1930 GMT).

In one video apparently of the attack, a shirtless man holding clear spray bottles in his hands is seen pacing as the grass in front of him burns.

He can be heard screaming "End Zionists!", "Palestine is Free!" and "They are killers!" towards several people in red t-shirts as they tend to a person lying on the ground.

Other images showed billowing black smoke above a park.

Redfearn told reporters that "initial callers indicated that there was a man with a weapon and that people were being set on fire," adding that police arrived "very quickly."

"When we arrived, we encountered multiple victims that were injured, with injuries consistent with burns and other injuries," he said.

Police on the scene "immediately encountered that suspect, who was taken into custody without incident," Redfearn said.

The Anti-Defamation League, a Jewish activist group, said on X that it was "aware of reports of an attack at today's Boulder Run for Their Lives event."

That event, it said, was a "weekly meeting of Jewish community members to run/walk in support of the hostages" who were seized during Hamas's unprecedented attack on Israel on 7 October 2023, sparking the war in Gaza. 

Israel's ambassador to the United Nations in New York voiced outrage at the attack.

"Terrorism against Jews does not stop at the Gaza border -- it is already burning the streets of America," he said in a statement.

"Today, in Boulder, Colorado, Jewish people marched with a moral and humane demand: to return the hostages. In response, the Jewish protesters were brutally attacked, with an attacker throwing a Molotov cocktails at them," he said.

"Make no mistake -- this is not a political protest, this is terrorism."