President Bush is sending his national security adviser to the Middle East next week to keep up pressure on Israel and the Palestinians to reach agreement on launching formal peace talks, a senior U.S. official said on Thursday.

The announcement came as Secretary of State Condoleezza Rice said she was encouraged by what she had heard from the two sides during four days of intense talks with Israeli and Palestinian officials and civic and business leaders in Jerusalem and Ramallah in the West Bank.

But Rice also acknowledged tensions between the two sides as they try to craft a joint statement that will be presented at a U.S.-hosted conference in late November or December at Annapolis, Md., where the United States hopes to announce the start of new formal peace negotiations to create a Palestinian state.

"I think they are very serious," Rice told reporters Thursday as she flew to London for talks with Jordan's King Abdullah. "The teams are serious. The people are serious. The issues are serious. So I am not surprised that there are tensions. I am not surprised that there are some ups and downs.

"That is the character of this kind of endeavor, but I was encouraged by what I heard," she added.

National security adviser Stephen Hadley will visit the Middle East next week to follow up on Rice's current round of diplomacy and Rice will return to the region for further discussions with Israelis and Palestinians at the end of the month or in early November, the U.S. official said.

The trip will also take Rice to an Iraq neighbors meeting in Istanbul, Turkey, said the official, who spoke on condition of anonymity because neither the Hadley trip nor Rice's return visit have been formally announced. The official did not give specific dates for the travel.

Hadley and Rice will press Israel and the Palestinians to bridge significant gaps on the substance of the conference's joint declaration, which would outline a way for the two sides to return to the negotiating table after seven years of bloodshed and diplomatic paralysis.

The Palestinians and their Arab allies such as Egypt and Jordan are insisting the document be detailed and specific with a timetable for formal peace talks, and the Israelis want language that is more vague.

The Palestinians want the document to include at least a sentence or two on how to solve each of the major issues of dispute, such as borders and Jerusalem, which both sides want to claim as their capital.

The Palestinians' core demand is that the future border between Israel and Palestine be based on the pre-1967 Mideast War lines, with modifications through land swaps. Israel captured the West Bank and other areas in the 1967 war.

On Wednesday, Rice called the conference a new "moment of opportunity" for the two sides, although she also cautioned that the going would be tough. Her comments came after she met twice with Israeli Prime Minister Ehud Olmert and Palestinian president Mahmoud Abbas over the course of several days.

While Rice was engaged in diplomacy on the ground, Bush told reporters in Washington that he was pleased with her progress and promised a staunch U.S. effort to make the conference a success.

"The reason why there needs to be a vision of what a state could look like is because the Palestinians that have been made promises all these years need to see there's a serious, focused effort to step up a state," Bush said.

He said he is seeking an Arab "buy-in" for a peace deal, something Rice is pressing for on her mission by meeting with Egyptian President Hosni Mubarak on Tuesday in Cairo and King Abdullah on Thursday.

Arab countries, notably U.S. allies Egypt, Jordan and Saudi Arabia, have been reluctant to commit to attending the conference unless there are guarantees that it will yield firm results.

Rice appeared to have won Egypt's backing. After her talks in Cairo, Egyptian Foreign Minister Ahmed Aboul Gheit endorsed the conference publicly less than 24 hours after suggesting it be postponed.

On Wednesday, ahead of Rice's meeting with King Abdullah, a senior Jordanian official echoed the initial Egyptian sentiments, saying the conference should be delayed if more time is deemed necessary.