On the eve of US President George W. Bush's visit to Israel, Palestinian Authority officials expressed pessimism regarding the chances of reaching an agreement with Israel before the end of this year.

PA President Mahmoud Abbas said Tuesday that the issue of the Palestinian refugees remained the most important issue.

"It's not less important than the issue of Jerusalem or the borders or other issues that are being discussed [with Israel]," he said during an event in Ramallah marking Nakba (Catastrophe) Day - the term Palestinians use to mark Israeli Independence Day.

"The Palestinian people won't forget the right of return for the refugees," Abbas said. "This is one of the rights of the Palestinian people. Our people have been living this nakba for 60 years, and they are continuing to struggle and defend themselves."

Abbas said the Palestinian negotiators were well aware of the rights of the Palestinians and would continue to stick to them.

"We are trying to achieve a just peace," he said. "We are serious in our efforts, and we want the support of the international community."

The head of the PA negotiating team, Ahmed Qurei, ruled out the possibility that a breakthrough would be achieved by the end of this year.

"We are holding serious negotiations [with Israel] on all the crucial issues," Qurei said. "All the issues are on the table and there are many differences between the two sides."

Qurei said that despite the difficulties, the Palestinians would continue to negotiate with Israel "without making any concessions or surrendering our rights. It's possible that the negotiations won't lead to anything, but this would not be the end of the road."

Nimer Hammad, political adviser to Abbas, said that without the "strong intervention" of the international community in the Palestinian-Israeli negotiations, it would be impossible to reach an agreement before the end of the year."

Hammad warned against the "marginalization" of the Israeli-Palestinian conflict in the wake of other events taking place in the Middle East, such as the latest crisis in Lebanon.

"As we have said since the Annapolis peace conference, unless there is a strong and effective intervention on the part of the international community, we won't be able to reach an agreement during the year 2008," he said.