* Kerry set to return to Washington for consultations
* Top U.S. diplomat calls for a "reality check"
* Palestinians, Israelis trade blame for impasse
Speaking during a visit to Morocco after a week of setbacks, Kerry said there was a limit to U.S. efforts if the parties themselves were unwilling to move forward.
"This is not an open-ended effort, it never has been. It is reality check time, and we intend to evaluate precisely what the next steps will be," Kerry said, adding he would return to Washington on Friday to consult with the Obama administration.
U.S. officials say Kerry had been blindsided by recent Israeli and Palestinian moves that had compromised undertakings made when they launched the latest round of talks aimed at ending their enduring conflict last July.
"They say they want to continue, neither party has said they have called it off, but we are not going to sit there indefinitely," Kerry said, making his bleakest assessment yet of talks that he has dedicated a huge amount of energy to.
The negotiations were catapulted into crisis at the weekend when Israel refused to act on a previously agreed release of Palestinian prisoners unless it had assurances the Palestinians would continue talks beyond an initial end-April deadline.
Kerry flew to Jerusalem to try to find a solution. Just when he believed a convoluted deal was within reach, Palestinian President Mahmoud Abbas signed 15 international treaties, making clear he was ready to beat a unilateral path to world bodies unless he saw more movement from the Israelis.
A senior Palestinian official, Nabil Shaath, told Reuters that Abbas had not intended to upset Kerry, but rather to shine a spotlight on Israel's failure to release the prisoners.
"I think (Kerry) will return because we have not abandoned the process," said the veteran negotiator, speaking in Ramallah, the Palestinians' administrative capital in the West Bank.
"We will continue these negotiations as we agreed, and I wish for once that America's patience runs out -- with Israel and not the Palestinians," he added.
With both sides looking to blame the other for the impasse, Israel's centrist finance minister, Yair Lapid, said he questioned whether Abbas wanted a deal, pointing to a lengthy list of Palestinian demands published on Maan news agency.
These included lifting a blockade on the Gaza Strip, and freeing a group of high profile prisoners, including Marwan Barghouti, jailed a decade ago over a spate of suicide bombings.
"(Abbas) should know that at this point in time his demands are working against him. No Israeli will negotiate with him at any price," said Lapid, one of the more moderate voices within Prime Minister Benjamin Netanyahu's rightist coalition.
Kerry has spent much of his first year as America's top diplomat invested in the Middle East peace process, and has visited the region more than a dozen times.
He broke off twice from his current 12-day trip in Europe and the Middle East to see Israeli and Palestinian leaders in an apparently effort to salvage the peace negotiations.
The talks have struggled from the start, stalling over Palestinian opposition to Israel's demand that it be recognized as a Jewish state, and over the issue of fast-growing Israeli settlements in the occupied West Bank and East Jerusalem.
Palestinians want an independent state in Gaza, the West Bank and East Jerusalem - lands captured by Israel in the 1967 war. While all parties say negotiations are the best path to peace, Palestinians say they may eventually resort to international bodies to force Israel to make concessions.