"Before one trip across enemy lines, resistance fighters told him that Jews were being murdered on a massive scale, and smuggled him into the Warsaw Ghetto and a Polish death camp to see for himself," Obama said.
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The Polish government keenly watches the global media for descriptions of former concentration camps as "Polish" because it says the term - even if used simply as a geographical indicator - can give the impression that Poland bore responsibility for Nazi Germany's World War II genocide.
Tommy Vietor, a spokesman for Obama's National Security Council, said that the president "misspoke."
"He was referring to Nazi death camps in Poland.
The Presidential Medal of Freedom ceremony on Tuesday (Photo: AFP)
"We regret this misstatement, which should not detract from the clear intention to honor Mr Karski and those brave citizens who stood on the side of human dignity in the face of tyranny."
A US official, speaking on condition of anonymity, recalled that Obama visited the Warsaw Ghetto Memorial during his visit to Poland last year and had repeatedly paid tribute to the bravery of Poles during World War II.
On several other occasions, Obama has also paid tribute to Poles who perished in Nazi camps like Auschwitz-Birkenau, near the Polish city of Krakow, and said the victims must never be forgotten.
In his remarks on Tuesday, Obama noted that Karski had taken his testimony about the horror of Nazi death camps and the Warsaw ghetto to wartime US president Franklin Roosevelt.
As a young man, Karski, who was a clandestine officer of the Polish government-in-exile in London, witnessed scenes of starvation and death after infiltrating the Warsaw Ghetto.
Dressed as a Ukrainian guard, he also went to a Nazi transit camp near Warsaw where he saw Jews beaten and stabbed and loaded onto train cars treated with quicklime for transit to the gas chambers.
Karski, who died in Washington aged 86 in 2000, later became a professor of history at Georgetown University.
The majority of the six million Jews who perished during the Holocaust were murdered in death camps set up and entirely controlled by Germany in occupied Poland.
A million of them died at Auschwitz-Birkenau, which has become the era's most enduring symbol.
Poland was home to Europe's largest pre-war Jewish population, some 3.5 million people. Polish Jews represented around half the Nazis' victims.
Around three million non-Jewish Poles were also killed over the six years that followed the Nazis' 1939 invasion, many of them in death camps.
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