He told an audience at The Screen On The Hill cinema in Hampstead that children should not have such subjects presented to them with a "spoonful of sugar".

Spielberg is executive producer of The Last Days, which is co-produced with his Shoah Foundation, set up to record the testimonies of Holocaust survivors.

It won an Oscar for best documentary earlier this year, but does not open in the UK until 8 October.

He said: "This is a lesson that does not need a spoonful of sugar to make sure it goes down sweetly. This is a lesson that must be learned painfully."

The film is part of Spielberg's plan to record the testimonies of Holocaust survivors

The film tells the story of five Hungarian Jews - now US citizens - during World War II. The five return to their home towns, where they tell their stories of deportations to concentration camps following the Nazi invasion of Hungary.

Holocaust survivors and members of the UK's Jewish community were invited to the screening, which also saw appearances by actors Richard Dreyfuss and Olympia Dukakis.

Spielberg hailed the film as an "extraordinary journey back in time", adding the only people who really understood the Holocaust were the people who survived it.

The Shoah Foundation has already collected 50,000 testimonies from people in 55 countries.

Although the film does not open in the UK until later this year, it can be seen at the Jewish Film Festival in London in August.

Schindler's List, which was released in 1993, was based on author Thomas Keneally's book about Oskar Schindler, a German factory owner who used his business to help save over 1,100 Jews from concentration camps. It won seven Oscars.