Huerta campaigned alongside fellow labour leader Cesar Chavez for farm workers' rights in the 1960s and 1970s, founding the United Farm Workers Union

US civil rights leader Dolores Huerta/Screengrab

US civil rights leader Dolores Huerta has died at the age of 96, her family has announced.

Through her foundation, Huerta had continued her activism work addressing immigration and voting issues.

"As you can imagine, this is an incredibly difficult time for us, and we ask for privacy as we grieve this incredible loss to our family," family spokesman Erik Olvera said on Wednesday.

Earlier this year, Huerta said Chavez, who died in 1993, had "manipulated and pressured" her into having sex and on another occasion forced her in the 1960s.

That was after the New York Times published an investigation detailing allegations from Huerta and two other women, who said Chavez groomed and sexually abused girls involved in the labour movement.

Huerta said the two encounters with Chavez resulted in pregnancies that she chose to keep secret, arranging for the children to be raised by other families.

Chavez's family said in a statement at the time that they were "devastated" by the reports and praised the women's "courage to come forward".

The UFW was among those paying tribute to Huerta on Wednesday, saying in a statement posted to social media on Wednesday that Huerta had coined its motto - "Si Se Puede!", Spanish for "Yes, we can!" - which became a familiar rallying cry for activists seeking better working conditions for Hispanic farm workers.

"Her life's work was to empower her community to face and overcome injustices, and she has left farm workers and the entire Latino community stronger and more organised than she found it," the UFW said.

"Her commitment to civic engagement and bringing political power to disenfranchised people is a lesson we all should carry to the ballot box."

Huerta also helped end what was known as the Bracero Program, where Mexican men were brought to work temporarily into the US, and helped negotiate the 1986 Immigration Reform Act, according to the union.

The president of the voting group Voto Latino, María Teresa Kumar, called Huerta a "civil rights giant" who gave "her entire life, until her final days, to championing and fighting injustice for everyone, no matter who they were or where they came from".