WASHINGTON (AFP) - More than 840 Holocaust survivors and some of the United States (US) soldiers who liberated them from Nazi death camps in World War II came together on Monday in what could be the last ever reunion of its kind.

They gathered under a large white tent by the National Mall for a 20th anniversary tribute to the US Holocaust Memorial Museum, one of the most visited attractions in Washington, with more than 1.6 million visitors a year.

Bill Clinton, returning to the institution he dedicated in 1993 when he was US president, said that even in a capital full of monuments "the Holocaust Memorial will be our conscience ... for now and forever."

With most Holocaust survivors and World War II veterans in their 80s or 90s, the museum opted not to wait until the usual 25th anniversary milestone to reassemble as many living witnesses to that dark period of history as possible.

Although it gets part of its funding from the US government, the museum - a global hub for Holocaust research with its rich archives - hopes to raise US$540 million (S$668 million) by 2018 to continue its work.

Honored guests on Monday included more than 100 US military veterans, some wearing their carefully preserved wartime uniforms, from units that liberated Nazi death camps as Allied forces swept through Europe.

The event also paid tribute to Marc Toureille, who as a young boy in the Herault department of southern France, helped his pastor father Pierre-Charles Toureille to spirit hundreds of Jews to safety in Spain and Switzerland.

Living in a railroad village deep inside Vichy France "we didn't know about the gas chambers, but we knew it would turn bad," 84-year-old Toureille, who now lives in Massachusetts, told AFP.

Nobel Peace Prize laureate Elie Wiesel, 84, the museum's founding chairman and a survivor of the Auschwitz concentration camp in Poland, appealed to younger generations never to forget.

"It is your memory that inherits ours," he said. "Our memory will live in yours. Remember that, young people, that you have an ideal ... the ideal of saving whatever the past has to offer for the future."

Six million European Jews were killed during the Holocaust and the museum also remembers others who suffered in their thousands from Nazi persecution, such as Roma and homosexuals.

Today, the specter of renewed genocide remains high on its list of concerns. Just last month, it published a report warning that genocidal acts are "a very real possibility" in Syria if the civil war there drags on.

For many of the survivors at Monday's tribute, despite their advanced age, memory was no problem at all.

"They were organized so well, the Germans, it was unbelievable. They had everything prepared," said Alan Zimm, 92, vividly recalling his internment in the Lodz ghetto in Poland and the Buchenwald concentration camp.

Still a working tailor in Richmond, Virginia, Zimm credits a German engineer - he never did learn his name - for slipping him sandwiches and survival tips during his time in forced labor at an underground V-2 rocket factory.

Zimm's wife Halina, 85, still hopes to connect with descendants of a family named Freiman, Freyman or Friman who gave her papers that enabled her to get through the war posing as a Roman Catholic named Wanda Kazusk.

"Of course, World War II was a tragedy, what happened to all the people in Europe," said Barbara Lau, 70, who came to Washington from Munich with thoughts of her Jewish mother-in-law shot by the Nazis.

"But the Holocaust was something which was planned. Do you understand? It was something which was done systematically, without exception."