Pop superstar Beyonce pleaded Thursday for an end to the "war" on minorities in the United States after another two contentious killings by police of African-Americans.

The singer, who has been increasingly outspoken on police brutality, provided a link to her legions of fans to contact US legislators to protest the deaths of motorist Philando Castile in Minnesota and Alton Sterling in Louisiana.

"This is not a plea to all police officers but toward any human being who fails to value human life. The war on people of color and all minorities needs to be over," Beyonce wrote in an open letter on her website and to her more than 77 million followers on Instagram.

Beyonce voiced fear that the killings, the latest in a series of racially charged shootings in the United States, would make minorities feel "helpless and hopeless."

"But we have to believe that we are fighting for the rights of the next generation, for the next young men and women who believe in good," she wrote.

"This is a human fight. No matter your race, gender or sexual orientation. This is a fight for anyone who feels marginalized, who is struggling for freedom and human rights."

Beyonce, who rose to stardom with catchy but largely apolitical hits such as "Crazy in Love" and "Single Ladies," aligned herself with the Black Lives Matter movement in a video earlier this year for her song "Formation."

The video, shot in New Orleans, features police officers with their hands in the air as if being arrested. She performed the song at the Super Bowl, the most watched US television event of the year, leading to criticism from some conservative commentators but also wide praise from activists.

Beyonce's sister, fellow singer Solange, also raised her voice after the latest killings.

Saying she wanted comfort when faced with the "images of slain black bodies," she released a raw video of herself on Instagram singing "Black Maybe," the 1972 song by Motown singer Syreeta written by her ex-husband Stevie Wonder.