President Obama has found himself embroiled in a diplomatic spat after he referred to Treblinka as a "Polish death camp" rather than noting that it was located on Nazi-occupied land.

The US President, addressing guests at an event marking the contribution of the Polish resistance hero Jan Karski, spoke of how Mr Karski had been smuggled "into the Warsaw Ghetto and a Polish death camp to see for himself" what was happening.

Tommy Vietor, a spokesman for the White House National Security Council, explained after that the president had "misspoke".

"He was referring to Nazi death camps in Poland," he said. "We regret the misstatement, which should not detract from the clear intention to honour Mr Karski and those brave citizens who stood on the side of human dignity in the face of tyranny."

But Polish officials present at the ceremony quickly complained that this was a "factually incorrect" way of describing the concentration camp, which was built in Poland only after the Nazi invasion of the country.

The slip was also labelled of an act of "ignorance and incompetence" by Polish Foreign Minister Radek Sikorski, who called for the White House to apologise for "this outrageous error".

"We cannot accept such words even if they are spoken by the leader of a friendly power - or perhaps especially in such situations," said Polish Prime Minister Donald Tusk. We expect diligence, care, and respect from our friends on issues of such importance as World War II remembrance."

Abraham Foxman, national director of the Anti-Defamation League, said the president's "unwitting mistake" was a reminder of a "perennial problem".

He said: "This only highlights the need for ongoing education about the history of the Second World War and the Holocaust."