President Barack Obama has written a letter to the Polish president expressing "regret" for an inadvertent verbal gaffe that caused a storm of controversy in Poland this week.

Obama on Tuesday used the expression "a Polish death camp" while honoring a Polish World War II resistance hero rather than wording that would have made clear that he meant a death camp that Nazi Germany operated on Polish soil during its wartime occupation of Poland.

Warsaw has been waging a campaign for years against phrases such as "Polish death camps" or "Polish concentration camps" to refer to Auschwitz, Treblinka and other German killing sites. The language deeply offends Polish sensitivities because Poles not only had no role in running camps such as Auschwitz, but were considered racially inferior by the Germans and were themselves murdered in them in huge numbers.

"In referring to `a Polish death camp' rather than `a Nazi death camp in German-occupied Poland,' I inadvertently used a phrase that has caused many Poles anguish over the years and that Poland has rightly campaigned to eliminate from public discourse around the world," Obama wrote. "I regret the error and agree that this moment is an opportunity to ensure that this and future generations know the truth."

Polish President Bronislaw Komorowski made the letter, dated Thursday, available to journalists in Warsaw on Friday, expressing satisfaction at Obama's words.

Komorowski called Obama's letter "a very important moment in the battle for historical truth."

Obama made the verbal slip-up while posthumously awarding the Medal of Freedom to Jan Karski, a resistance fighter who struggled to tell the outside world about the murder of Jews in his country. He smuggled himself into the Warsaw Ghetto and a death camp, witnessing the atrocities committed against the Jews firsthand. He then took that information to President Franklin Roosevelt, imploring the world to act.

Karski later became a professor at Georgetown University and died in 2000.