Secretary of State John Kerry Sunday rejected Israeli and Palestinian claims the US was biased as he made a whistlestop tour of allies Jordan and Saudi Arabia to woo support for his peace plan.
Kerry promised that any plan he came up with would be "fair and balanced" and likened his efforts to broker a compromise between the conflicting demands of the two sides to a puzzle.
"In the end all of these core issues fit together like a mosaic, like a puzzle and you can't separate out one piece or another," Kerry said in Jerusalem before flying to Amman, and later Saudi Arabia, seeking backing for his proposals.
Each piece was interlinked, he stressed, and depended on the compromises the other side might be prepared to make.
"And there's always tension as to when you put your card on the table as to which piece you're willing to do, when and how," said Kerry.
But he warned his efforts could ultimately fail, saying he could not tell when "the last pieces may decide to fall into place, or may fall on the floor, and leave the puzzle unfinished. That's exactly what makes this such a challenge."
Kerry has already spent three intense days shuttling between Israel's Prime Minister Benjamin Netanyahu and Palestinian president Mahmud Abbas.
On Sunday, he held more than an hour of talks with Jordan's King Abdullah II before flying on to Saudi Arabia for talks with King Abdullah at his desert residence outside Riyadh.
Jordan borders the occupied West Bank and under its 1994 peace treaty with Israel is recognized as playing a historic role in the guardianship of Muslim sites in east Jerusalem.
Saudi Arabia was the author of a 2002 Arab League peace initiative.
Kerry is on his 10th visit to the region since taking office less than a year ago, and kicked-started nine months of direct negotiations in July after a three-year hiatus.
But bitter recriminations between the two sides, who have refused to budge on their mostly irreconcilable demands, have burst into the open during this trip.
Israeli and Palestinian officials have both said the emerging proposals appear to favour the other side.
"I can guarantee all parties that President Obama and I are committed to putting forward ideas that are fair, that are balanced, and that improve the security of all the people of this region," Kerry insisted.
Refusing to outline any specific details, Kerry urged Israeli and Palestinian leaders to make "tough choices" to bring about a lasting peace after decades of conflict.
"We are now at a point where the choices narrow down, and the choices are obviously real and difficult."
Kerry has pledged to work even more intensively in the coming months, seeking first to agree a framework to guide the talks with an April deadline looming.
A peace treaty would deal with all the core issues dividing the two sides, including the contours of a future Palestinian state, refugees, the fate of Jerusalem claimed by both as a capital, security and mutual recognition.
Kerry insisted Saturday there had been some progress, although more work was needed.
A cabinet member close to Netanyahu said Sunday Israel rejects any US-proposed security concessions for the Jordan Valley, where the West Bank borders Jordan.
"Security must remain in our hands. Anyone who proposes a solution in the Jordan Valley by deploying an international force, Palestinian police or technological means... does not understand the Middle East," said Intelligence Minister Yuval Steinitz.
The comments came after a Palestinian source said Washington was proposing a mixed Israeli-Palestinian military presence to ensure security in the area, without setting a deadline for Israeli troops to withdraw.
The Palestinians want an international security force to patrol the Jordan Valley under a peace deal, but Israel insists on maintaining a long-term military presence on the territory it currently occupies.
The Israeli premier also took issue Sunday with the Palestinians' refusal to recognize Israel as a Jewish state.
"The Palestinians are continuing their campaign of inciting hatred, as we have seen in the last few days with their refusal to recognize Israel as a state for the Jewish people," Netanyahu told his cabinet.
The Palestinians argue any such recognition might compromise the rights of Israeli Arabs and the "right of return" they claim for Palestinian refugees.