John Kerry's high-stakes gamble that he could finally achieve the dream of generations and bring peace to the Middle East seems to be collapsing as easily as a house of cards.

Despite a dozen visits to Israel and the West Bank since he became US secretary of state 14 months ago and many more late-night meetings with his recalcitrant partners in capitals around the world, it appears after all that he may have been trumped.

While there was always a certain hubris to his mission impossible, the political dangers facing wily Israeli Prime Minister Benjamin Netanyahu conspired this week with decades of pent-up anger among Palestinians to throw up the most serious crisis to the fragile peace negotiations since they resumed in July.

Yet at the start of Kerry's latest overseas trip there was little to suggest he would return to the US 13 days later with his peace effort in trouble and a blunt admission that he and the White House needed to "evaluate" the next steps.

Indeed, Kerry had not visited Israel in three months in a tacit recognition that each trip raised expectations and usually triggered some kind of provocative move from one of the parties.

His monthly commute between Washington and Jerusalem had also begun to raise eyebrows with little tangible progress to show and an April 29 deadline looming.

Israeli Defense Minister Moshe Yaalon had called Kerry "obsessional" and "messianic" and at home some critics said he was "delusional."

- 'Reality-check time' -

All roads to Jerusalem are littered with past failed peace negotiations which have wound through places such as Madrid, Oslo and Camp David.

But this time Kerry felt there was something within his grasp, a deal under which both sides would agree to keep talking into next year, as some of the nitty-gritty contours of a pact began to emerge.

He deeply believes that a comprehensive peace treaty is the only way to secure Israel's future and build a better tomorrow for Palestinians, with both peoples having suffered too much.

So the 70-year-old former senator, the son of a diplomat, stepped willingly into the quagmire that is Middle East peace.

He has invested huge amounts of energy, setting a punishing schedule which would defeat many half his age and remaining eternally optimistic and unflappable even after hours locked in tense negotiations.

It was sobering therefore on Friday as he prepared to head home -- after the Israelis canceled the last prisoner releases and the Palestinians said they would seek statehood at 15 agencies at the UN -- that in a rare moment of frankness and frustration he admitted "it's reality-check time."

"There are limits to the amount of time and effort that the United States can spend if the parties themselves are unwilling to take constructive steps in order to be able to move forward," Kerry told reporters in Rabat.

With the war in Syria, Iran's nuclear program and the crisis in Ukraine, "we have an enormous amount on the plate," he said.

- No-one walking away yet -

Exactly what Kerry's next move will be remains uncertain, and he has insisted that the negotiators remain at work on the ground.

But it's more than possible that he'll give both sides a little space to figure out what they want to do, as he huddles with the White House.

There will be another three-way meeting likely on Sunday in the region to assess the way forward, officials close to the talks say, and the US insists the negotiations are not dead.

Only a few months ago, Kerry's stock had been rising with his brand of face-to-face diplomacy winning praise.

He had helped kick-start the peace talks after a three-year gap, sealed a deal with Russia to rid Syria of its chemical weapons, and negotiations with Iran over its suspect nuclear program had made the first progress in a decade.

Now critics will be sharpening their pencils in glee.

But he has three more years in office, and almost boundless patience.

The White House Friday defended the "tireless" Kerry, saying his long-odds Middle East peace bid had not been a waste of time because the stakes were so high.

But Josh Earnest, a White House spokesman, admitted that the chances of Kerry succeeding when he embarked on his Middle East peace quest a year ago had never been high.

"I don't know if people in Las Vegas are betting on these kinds of things these days, but I'm sure the odds... would be very long."

Earnest refused to say that Washington had given up.

"That presupposes an additional step here, that at some point somebody throws up their hands and walks away. Secretary Kerry's certainly not willing to do that."