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Albert Einstein in his study in Princeton, New Jersey, 1951.

A letter from Albert Einstein warning of the persecution of Jews in Germany on the eve of World War II is up for auction in California, with the sale ending Tuesday evening Pacific time.

The physicist writes of the importance of "rescuing our persecuted fellow-Jews from their calamitous peril and leading them toward a better future" in the June 10, 1939 letter.

Einstein praises New York businessman Hyman Zinn for his "splendid work" on behalf of refugees.

"We have no other means of self-defense than our solidarity and our knowledge that the cause for which we are suffering is a momentous and sacred cause," Einstein writes to Zinn, of the Manhattan Button Company.

The typewritten letter, hand-signed "A. Einstein," was written just under three months before the outbreak of World War II, when the persecution of Jews was already well underway.

Auctioneer Nate Sanders said he expects the letter to go for $5,000 to $7,000.

It "contains powerful content showing that Einstein was devoted to the Jewish people," Sanders said in a statement.

Bidding stood at just over $3,000 for the letter with 12 hours left in the auction.

It closes at 5 p.m. Pacific time on Tuesday.

The lot also includes the envelope the letter was sent in.

An estimated six million Jews died at the hands of the Nazis and their allies in the Holocaust.

Einstein was born in Germany but renounced his citizenship in 1933, when Adolf Hitler became leader of Germany, and moved to the United States.