US Middle East envoy George Mitchell arrived here Thursday for a new round of shuttle diplomacy aimed at restarting Israeli-Palestinian talks, amid President Obama’s admission that he had been too optimistic about the prospects for peace.

Mr. Mitchell, here for the first time since last November, met with Israeli Defense Minister Ehud Barak in Tel Aviv and was scheduled to hold talks later Thursday with Prime Minister Benjamin Netanyahu and on Friday with Palestinian Authority President Mahmoud Abbas.

"Recognizing the complexities and difficulties outstanding, we will pursue until we achieve that objective," Mitchell told reporters. "That's my purpose here today."

But observers say that the sides seem more entrenched in their positions.

"I don't think there's any sense of an upcoming breakthrough," says Jonathan Spyer, an expert on the Israeli-Palestinian conflict at the Interdisciplinary Center in Herzliya, north of Tel Aviv.

He suggests that disappointment with the level of US involvement, together with internal Palestinian and Israeli political issues, are blocking progress.

"There's no urgency on the Palestinian side," Mr. Spyer posits. "They're not willing to be seen as making concessions that will be criticized by Hamas. It seems the Palestinians don't need or want negotiations with Israel. They're doing quite nicely against Israel in international forums, and economic life is better than it has been in while.

"Israel has positioned itself as ready to start negotiations, but it isn't going to agree to a more extensive settlement freeze that includes East Jerusalem," he says. "The parties to the conflict right now have no reason to move further than they have."

Adding to dampened expectations, US President Barack Obama said in a Time interview published Thursday that his administration "overestimated” its ability to persuade both sides to negotiate, and added that he’d hoped for bold moves.

Mr. Netanyahu told foreign reporters on Wednesday that even if Palestinians obtained their own state, Israel would insist on controlling West Bank border crossings to “effectively stop the infiltration of rockets and other weapons.” Defining such plans for the first time, he implied that Israel would stay in control of the western half of the Jordan River Valley. The Jordan River separates Israel and the West Bank from Jordan.

Generations of Israeli security hawks have taken the same position.

“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. What are these if not Israeli preconditions on negotiations that erode any foundation of hope for the two-state solution?” chief Palestinian negotiator Saeb Erakat said Thursday.

“We had hoped to hear a clear commitment to negotiations without preconditions. What we got instead was Mr. Netanyahu again trying to dictate their terms and preempt their outcome. The only remaining obstacle to negotiations are the conditions Mr. Netanyahu continues to impose.”

But the US Embassy says Mitchell will not relent, tirelessly pursuing peace as he did in Northern Ireland. "He's just trying to persuade, and sometimes you persuade by restating your position, again and again and again, until you get an agreement," said spokesman Kurt Hoyer.