Snide Israeli criticism of John Kerry's Middle East peace bid has finally triggered US anger after a weeks-long whispering campaign, but his secretive, solo diplomacy has exposed him to attack.

As Kerry has shuttled between Palestinian and Israeli leaders, seeking painful compromises from both sides, he has faced growing personal attacks portraying him as well-meaning but naive.

With few details emerging from his closely-guarded talks, both sides have vented their frustration in the media, flinging mud at the US secretary of state in a bid to ensure their point of view is heard.

But hardline Israeli Defense Minister Moshe Yaalon seems to have gone a step too far on Tuesday when he branded Kerry's quest an "incomprehensible obsession" and described him as a man with a "sense of messianism."

In an unusually robust response to Washington's closest regional ally, the White House called the remarks "offensive and inappropriate" and a State Department official demanded an apology.

On Wednesday, Kerry vowed not to let the latest Israeli slur derail his effort to build on the momentum created when he pushed the two sides back to the negotiating table to end a three-year stalemate.

"I am going to work with the willing participants who are committed to peace and committed to this process," he said, during a trip to Kuwait.

But in many ways Kerry has fallen to victim to his own insistence on complete secrecy, giving both sides license to use him as a punching bag with no public information to counter their attacks.

Despite having appointed former US ambassador Martin Indyk to shepherd negotiations on the ground, Kerry has made 10 trips to the region and has become the very public face of the quest for peace.

With so much at stake "especially in Israel where politics are so visceral and open, it will only get nastier before it gets better," said Daniel Kurtzer, former ambassador to Israel.

But, he argued, Kerry's approach to "keep the diplomacy insulated until you're ready to make the case" was the best one when faced by six decades of corrosive hatred.

"I think Kerry is getting close to the nitty-gritty," Kurtzer told AFP, adding "he's clearly challenging both sides" on the hardest issues such as borders, security and the fate of Jerusalem.

There is resistance within Prime Minister Benjamin Netanyahu's right-wing government to Kerry's proposals for future security arrangements between a Palestinian state and neighboring Jordan.

And a senior official in Netanyahu's Likud party said the defense minister had merely vocalized what Netanyahu himself was thinking.

But analyst Aaron David Miller, who worked with six secretaries of state on Middle East policy, said he did not see the comments by Yaalon -- "a maverick, who speaks his mind" -- as presaging a crisis.

Such outbursts arise "episodically" in the US-Israeli relationship, and reflect the tensions within Netanyhau's cabinet, said Miller, a distinguished scholar at the Wilson Center think-tank.

He agreed, however, that the peace talks "would not be happening without Kerry, for good or ill, and he'll have to accept the consequences."

At some point Kerry will "have to bring his efforts above ground" and let people see if an agreed framework for talks can lead to a full peace treaty, which would also depend on support from President Barack Obama.

"Yes, this is John Kerry's peace process, but we don't know where the president is on this," Miller said.

While it is probably correct to assume that Obama backs Kerry, success will also depend on what hard decisions are requested from Israel.

"If those decisions involve stressing the Israelis, at a time when they're stressing them on Iran, it's not altogether clear to me that the president is interested in this."

He predicted Kerry was likely to reach an agreed framework by an April deadline, "but the key is, is that piece of paper the key to an empty room or is it something will actually lead to an agreement?

"That's the question."