An undated photo shows Franz Kafka, author of The Trial and The Castle. His friend Max Brod saved his papers from the Nazis. (Associated Press)

Previously unseen documents from the estate of 20th-century writer Franz Kafka could soon be released after being hidden in a Tel Aviv apartment for decades.

The papers include postcards, sketches and other documents taken out of Czechoslovakia by Kafka's friend and executor Max Brod.

Brod had defied Kafka's request to burn the papers after his death. The Czech-born German writer, author of The Trial, died in 1924 from tuberculosis in a sanatorium near Vienna.

Brod died in 1968 and left the papers to his former secretary and lover Esther Hoffe, who died last week at age 101.

Hoffe had been resisting attempts by the Israeli government to release the papers, saying she feared someone would try to steal them.

She sold the manuscript of The Trial at auction in London in 1990 for $2 million.

She also sold Kafka's diaries to a German publisher in the 1980s, but failed to hand them over.

Hoffe spurned attempts from both journalists and literary scholars to make the papers public, despite witnessing Kafka's growing stature as a 20th-century thinker.

The Israeli government once detained her from a flight out of the country, saying she was smuggling cultural properties. She was found to have Kafka's letters with her.

Her death may end the feud with Israeli bureaucrats, but it is far from clear what will happen to the papers.

Her daughters, Ruth and Hava, both in their 70s, now inherit the documents and have the right to determine their fate.

But the German publisher has a claim, and Israel says it will step in if necessary to preserve the papers as cultural artifacts. There is concern that the works will have deteriorated in Hoffe's damp Tel Aviv flat.

'Hidden treasure' tantalizes scholars

Scholars of Kafka are keen to study the documents, which may include unpublished works by a man considered one of the most influential literary figures to emerge from 20th-century Europe.

Israel has said it is prepared to make the documents available for literary study.

Brod was Kafka's greatest defender and promoter and helped bring his works, which included The Metamorphosis and The Castle, to publication after his death.

Brod fled Prague with two suitcases of Kafka's papers ahead of the Nazis in 1939.

In 1961, he donated the bulk of the papers to the Bodleian Library at Oxford University in the U.K., however he kept the manuscript of The Trial, which Hoffe later sold, and other papers.

Most of Kafka's Jewish family, including his three sisters, died in concentration camps.

Researchers are hoping the papers will throw new light on Kafka and his friendship with Brod, his greatest champion.

"This is a hidden treasure that the entire civilized world would be happy to discover," said Dr Zohar Maor, Bar-Ilan University, in Tel Aviv quoted in the London Telegraph. "Its value is incalculable."