After his meetings with the Israelis, Mitchell is scheduled to hold talks with the Palestinians in Ramallah on Friday.

Earlier on Thursday, Saeb Erakat, the chief Palestinian negotiator, accused Binyamin Netanyahu, the Israeli prime minister, of imposing "further conditions on negotiations" and announcing "Israel's intention to continue its occupation" of the West Bank whatever happens.

"Benjamin Netanyahu has said 'No' to a settlement freeze, 'No' to sharing Jerusalem, 'No' to the 1967 borders, 'No' to the rights of Palestinian refugees. Now he wants to retain the Jordan Valley," Erekat said in a statement.

He was referring to a comment by Netanyahu on Wednesday that Israel would retain military control around any future Palestinian state that included the West Bank.

"We had hoped to hear a clear commitment to negotiations without pre-conditions. What we got instead was Mr Netanyahu again trying to dictate their terms and preempt their outcome," Erekat said.

War of words

Addressing the foreign press late on Wednesday, Netanyahu attacked the Palestinian leadership for rejecting US calls to relaunch negotiations suspended for over a year.

"The Palestinians have climbed up a tree," he said. "And they like it up there. People bring ladders to them. We bring ladders to them. The higher the ladder, the higher they climb."

Critics said Israel had placed another obstacle in Mitchell's path by agreeing to upgrade to university status a college built in a Jewish settlement in the West Bank.

The decision by Ehud Barak, the defence minister, formalising a 2005 cabinet ruling, coincided with Mitchell's visit.

Erekat said it was "part of the same policy of dictation rather than negotiation".

"Every time Senator Mitchell comes to the region, they greet him with such policies," he said.

Diplomats say Mitchell seems to be seeking a face-saving way for Mahmoud Abbas, the Palestinian president, to drop his insistence that Netanyahu must stop all illegal settlement building in the West Bank and East Jerusalem before negotiations can be resumed.

Obama's admission

The US president, meanwhile, has acknowledged that he had underestimated the difficulty of resolving the Middle East conflict and had set his expectations too high in his first year.

"The Middle East peace process has not moved forward. And I think it's fair to say for all our efforts at early engagement, is not where I want it to be," Barack Obama told Time magazine in an interview published on Thursday.

"This is just really hard ... This is as intractable a problem as you get.

"... If we had anticipated some of these political problems on both sides earlier, we might not have raised expectations as high."

Obama said: "I think the Israelis and Palestinians have found that the political environment, the nature of their coalitions, or the divisions within their societies were such that it was very hard for them to start engaging in a meaningful conversation.

"And I think that we overestimated our ability to persuade them to do so when their politics ran contrary to that."