Senior U.S. diplomats have returned to the Middle East for an unannounced visit to try to find a way to restart Israeli-Palestinian peace talks that collapsed last year and now face new challenges.

Dennis Ross and David Hale's visit, confirmed by an Israeli official Wednesday, is their first to the region since special Mideast envoy George Mitchell resigned last month after failing to break the negotiations deadlock.

The first big challenge for the U.S. envoys is to find a formula for talks that would entice the Palestinians to drop their bid for unilateral recognition of a Palestinian state at the United Nations in September. The Palestinians, who hope to establish a state in the West Bank, east Jerusalem and Gaza Strip, adopted that strategy in frustration at the impasse.

"There are talks going on to see if there's a formula that will allow for the restarting of the talks that would cause the Palestinians to abandon their unilateral approach," said another Israeli official.

Both Israeli officials spoke on condition of anonymity because the visit not announced publicly.

Peace talks launched in September after a two-year breakdown collapsed just three weeks later after an Israeli moratorium on Jewish settlement building in the West Bank expired. The Palestinians refused to negotiate without a new moratorium that would also include east Jerusalem, and Israel would not yield to that demand.

Since then, the Palestinians have forged ahead with plans to seek U.N. recognition for a state with or without a peace deal with Israel. That effort faces major obstacles, not least because the United States, which opposes it, has veto power on the Security Council.

The U.S. has not indicated whether it would use the veto, but President Barack Obama has said the unilateral campaign for U.N. recognition would not help peace efforts in the region.

Also complicating any return to negotiations is a newly forged reconciliation agreement between Palestinian President Mahmoud Abbas' Western-backed Fatah group, which governs in the West Bank, and the Hamas militant group that rules Gaza.

Israel has said it would not negotiate with a Palestinian leadership that includes Hamas, which doesn't recognize Israel's right to exist and has killed hundreds of Israelis.

Both Israel and the U.S. consider Hamas to be a terrorist group.