The 65 documents, which were found in a New York archive in 2005, show how Otto Frank tried desperately, but ultimately in vain, to flee the Netherlands, where the Nazis had already started deporting Jews to concentration camps.

"As far as I can see, the USA is the only country we could go to. Perhaps you remember that we have two girls. It is for the sake of the children mainly that we have to care for," he wrote in one chilling letter.

"Our own fate is of less importance," he added.

Frank's youngest daughter, Anne, went on to chronicle the details of her teenage life hiding from the Nazis in Amsterdam from 1942 until 1944, when she was arrested and transported to a concentration camp, where she later died.

Had she escaped, "Anne Frank could be a 77-year old woman, possibly a writer, living in Boston today," said Richard Breitman, professor of history at the American University, on Wednesday as he revealed the existence of the letters at New York's YIVO Institute for Jewish Research, where they were found.

Cuban visa granted

For the institute, the discoveries are of deep significance: "What hasn't been known is how desperately her father, Otto, tried to get his family out of the Nazi-occupied Netherlands," the institute said in a statement.

Three of the documents written by Otto detail his attempts to get his family to safety in the United States. He also applied for a Cuban visa, which was granted, although it is not known if he ever found out.

In one letter, he wrote to an American friend, Nathan Straus, son of the founder of the Macy's retail empire, asking him to pay the fee for the visas.

"I would not ask if conditions here would not force me to do all I can in time to be able to avoid worse," Otto wrote.

The rest of the letters detail the abortive attempts by Straus and Otto's brother-in-law in the United States to get the Frank family US visas.

Good connection not enough

US immigration officials at the time had imposed strict restrictions due to national security concerns and fears of spying, even though the United States was not at war at the time.

"The Frank family had good connections. Still it was not enough... in 1941, it was late, and it turned out to be too late," Breitman said.

Otto in the end decided to hide his family in the attic of his company's building in Amsterdam, where they hid for two years until they were betrayed and deported. Only Otto survived the Holocaust, freed from Auschwitz in 1945.

It was only when he returned to Amsterdam that he found Anne's diary, which was published in 1947. The diary is one of the widest-read books in the world and made Anne one of the best-known casualties of the Holocaust.

"With the release of the file, the plight of the Franks becomes even more poignant, since the family was unable to escape even with the help and support of a prominent American," Yivo's director Carl Rheins said.

The documents were filed by the US National Refugee Service and were only discovered in mid-2005 by an archivist who stumbled across the letters because their cover file was unusually not marked with a date of birth.