U.S. President Barack Obama has expressed “regret” for what he called an inadvertent verbal gaffe that caused a storm of controversy in Poland.

Earlier this week, Mr. Obama used the term “Polish death camp” while honoring a Polish World War Two resistance hero, instead of referring to the camp as one operated by the Nazis in Poland during the war.

Mr. Obama voiced his regret in a letter to Polish President Bronislaw Komorowski, saying there is a chance to ensure that “this and future generations know the truth” about the death camps.

Mr. Komorowski praised the letter.

Polish officials are sensitive about inferences that the Poles operated concentration camps during World War Two. Camps such as Auschwitz and Treblinka were on Nazi-occupied Polish soil, but Poland had no role in running them.

Mr. Obama erred while posthumously awarding the Presidential Medal of Freedom to Jan Karski, who gave the Allies eyewitness accounts of the Nazi genocide against European Jews.

The White House expressed regret for what it called a misstatement, but Polish leaders urged Mr. Obama to further correct the record.

The Nazis killed 6 million Jewish people during the Holocaust, including some 3 million Polish Jews.