Law enforcement officials in California said Monday they were investigating the death of a Jewish man who died after an altercation at dueling pro-Israel and pro-Palestinian rallies.

The Ventura County Sheriff's Office said it had been called after a confrontation at the rallies on Sunday in Thousand Oaks, northwest of Los Angeles.

"Responding deputies located (Paul) Kessler, who was suffering from a head injury," a statement said.

"Witness accounts indicated that Kessler was involved in a physical altercation with counter-protestor(s).

"During the altercation, Kessler fell backwards and struck his head on the ground."

The 69-year-old died from his injuries on Monday, authorities said.

"The Ventura County Sheriff’s Office is investigating the incident and has not ruled out the possibility of a hate crime," the statement said.

Footage on social media appeared to show Kessler lying on the ground clutching his head.

The local Thousand Oaks Acorn newspaper quoted a police officer saying the majority of people present at the rally had been supporters of the Palestinians, but a small number of pro-Israel supporters also turned up.

The incident comes a month after Hamas launched a vicious assault on Israel from the Gaza strip, killing 1,400 people, mostly civilians, and taking over 240 hostages, according to Israeli authorities.

Israel's retaliation, which has involved an extensive bombing campaign and an intensifying ground invasion of the besieged Gaza Strip, has now claimed more than 10,000 lives, according to the Hamas-run health ministry.

Calls by Arab nations for a ceasefire and an end to the blockade of Gaza have been rejected by Israel which says it is fixed on neutralizing Hamas and rescuing the hostages.