The US secretary of state, John Kerry, has apologised for warning that Israel risked becoming an "apartheid state" if it did not reach a peace deal with the Palestinians, following a barrage of criticism in America.

In a statement, Kerry hit back at what he described as "partisan political" attacks against him, while stating that in retrospect he would have chosen a different word.

He said that apartheid was "a word best left out of the debate [in the US]" despite the fact that there have been similar warnings from senior Israeli politicians.

Kerry insisted his remarks were only an expression of his firm belief that a two-state solution was the only way to end the long-running conflict.

Kerry's reported comments, and his retraction, come at a sensitive time for the peace process, suspended by Israel last week after the agreement between the Palestine Liberation Organisation and Hamas to reconcile and try to form a unity government.

The US state department statement was released just hours before the deadline set nine months ago for the conclusion of this phase of the Israeli-Palestinian peace talks, which have come to an end without progress, and amid predictions that Kerry would concentrate on other international issues.

Without denying he used the word "apartheid", the statement insisted on Kerry's history of support for Israel over the past 30 years.

"I will not allow my commitment to Israel to be questioned by anyone, particularly for partisan, political purposes, so I want to be crystal clear about what I believe and what I don't believe," Kerry said after members of Congress and pro-Israel groups criticised him, with some demanding his resignation or at least an apology.

"First, Israel is a vibrant democracy and 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," he said.

"Second, 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," Kerry said.

The Daily Beast reported on Sunday that Kerry had made the remarks referring to the risk of Israel becoming an "apartheid state" at the closed door meeting on Friday.

According to reports, his remarks were apparently recorded after a reporter, who was not invited to the event and so not bound by the meeting's non-attribution agreement, managed to get in.

Kerry has invested significant time and energy in bringing the two sides to the negotiating table with the goal of reaching a deal, but has blamed both the Israeli and Palestinian leaders for a lack of progress.

That deadline expires on Tuesday with the parties having failed to reach an accord, a less ambitious framework deal or even an agreement to extend the negotiations.

"There is unlikely to be any diplomatic movement for months," said one Israeli official. "Perhaps until September at the earliest. He added it would be reasonable to expect both sides to concentrate on "minimising damage".

While Israeli political leaders have not commented publicly about Kerry's remarks, the secretary of state has faced fierce criticism in the US.

The Republican leader of the House of Representatives, Eric Cantor, said Kerry should apologise, while the American Israel Public Affairs Committee described his use of the term as offensive. Another pro-Israel lobby called for Kerry to resign, a call echoed by Republican senator Ted Cruz of Texas in a speech on the Senate floor.

Democratic senator Barbara Boxer was also critical of Kerry,, saying in a tweet: "Israel is the only democracy in the Middle East and any linkage between Israel and apartheid is nonsensical and ridiculous."