Speaking in Morocco, Mr Kerry said there were "limits" to the time Washington would expend on trying to get the sides to reach an agreement.

He spoke after steps taken by Israel and the Palestinians in the past two days which each side said violated previous promises.

Talks are meant to conclude next month.

Mr Kerry has shuttled backwards and forwards for negotiations with Israel and the Palestinians in recent weeks.

However, Washington has expressed exasperation at what it calls "unhelpful, unilateral actions" taken by both sides.

On a visit to Rabat on Friday, Mr Kerry said it was time to pause and consider where the process was going.

"This is not an open-ended effort, it never has been," he said. "It is reality check time, and we intend to evaluate precisely what the next steps will be."

The talks, which resumed in July under US auspices after a three-year hiatus, appeared to be on the point of breaking down this week, with Israel and the Palestinians blaming the other.

The Palestinians were furious when Israel did not sanction the release of a fourth batch of Palestinian and Israeli-Arab prisoners, as agreed in principle under the terms on which the Palestinians returned to peace talks last year.

A crowd of Palestinians threw missiles at Israeli soldiers outside a military prison near Ramallah.

Israel stressed that it had predicated any release on progress being made in the negotiations and on the Palestinians abiding by a commitment not to seek membership of international agencies.

Cabinet members also said they would block a release unless the Palestinians agreed to extend the talks beyond 29 April, the date by which the US had said it had hoped to reach a full agreement.

The Palestinians however said they would not agree to extend the talks unless the prisoners were freed and accused Israel of reneging on the deal.

Palestinian protesters demonstrated in the West Bank cities of Bethlehem and Ramallah on Friday over Israeli's decision not to free the prisoners.

There were clashes outside a military prison in the town of Beitunia, with Israeli soldiers using tear gas, stun grenades and rubber bullets to disperse a crowd of Palestinian youths who had thrown missiles at them.

On Wednesday, Palestinian Authority President Mahmoud Abbas signed applications to join 15 international conventions which Palestinian officials said was a response to Israel's failure to release the prisoners. Israel fears the Palestinians will use the treaties as a legal tool against it and to further enhance statehood which is subject to the negotiations.

It officially cancelled the prisoner release on Thursday in response to Mr Abbas's move.

Despite the acrimony, Mr Kerry said that neither side was abandoning the process.

"Both parties say they want to continue, neither party has said they want to call it off, but we're not going to sit there indefinitely," he said.