John Kerry has denied he has ever called Israel "an apartheid state", amid a row over comments reportedly made during a private meeting.
"I do not believe, nor have I ever stated, publicly or privately that Israel is an apartheid state or that it intends to become one," the US secretary of state said in a strong statement after calls for him to resign or at least apologise for the alleged comments.
"Anyone who knows anything about me knows that without a shred of doubt," said Kerry, according to the AFP news agency.
But Kerry, whose efforts to broker peace between Israel and the Palestinians have collapsed, did say that he had used a poor choice of words during his speech on Friday to international experts of the Trilateral Commission.
"I have been around long enough to also know the power of words to create a misimpression, even when unintentional, and if I could rewind the tape, I would have chosen a different word to describe my firm belief that the only way in the long term to have a Jewish state and two nations and two peoples living side by side in peace and security is through a two-state solution," he said.
"I will not allow my commitment to Israel to be questioned by anyone, particularly for partisan, political purposes."
The US-based Daily Beast online news site reported that Kerry had warned that "a unitary state winds up either being an apartheid state with second-class citizens - or it ends up being a state that destroys the capacity of Israel to be a Jewish state".
The website said it had been given a recording of Kerry's speech, which sparked a furore in Israel and led one Republican senator to call for his resignation.
Kerry has "repeatedly demonstrated a willingness to countenance a world in which Israel is made a pariah," said Senator Ted Cruz.
He said that Kerry should offer his resignation "before any more harm is done to our national security interests and our critical alliance with the state of Israel".
John McCain, a veteran Republican senator, also said Kerry should clarify his comments and apologise. He disagreed that Kerry should resign, however.
A US-imposed deadline for progress on Palestinian-Israel peace talks ran out on Tuesday. Last week, Israel pulled out of talks after Fatah announced it would seek to create a unity government with the Hamas group, which controls Gaza.