A day after the two main Palestinian factions announced surprise plans for a unity government, the challenge of bringing together two rival parties with distinct ideologies burst into view, with each side presenting a different picture of what the accord means and what produced it.

Mahmoud Abbas, the Palestinian Authority president, said Thursday that because he was also chairman of the Palestine Liberation Organization he remained in charge of peace efforts with Israel. The future unity government, he said, will have only two functions, to rebuild Gaza and set up elections within a year.

“The new government and peace talks are two different things,” Mr. Abbas told a group of Israelis who signed what they called the Israeli Peace Initiative this month and were invited to his headquarters for lunch. He said no activists of either his party, Fatah, or of the Islamists of Hamas would serve in the new government.

He added that negotiations with Israel were his preferred path to statehood rather than recognition from the United Nations, which he is also pursuing. But he repeated that for negotiations to begin, he needed a moratorium in Israeli settlement building in the West Bank, a condition that Israel rejects.

Aides to Mr. Abbas, who were not authorized to speak publicly, said one central reason the two sides were reconciling after four years of enmity was that Hamas had suddenly found itself in a position of weakness. Hamas is based in Syria, which is in turmoil, and it may not be able to stay there over the long term. Moreover, the aides said, Egypt may be friendlier to Hamas than it was under President Hosni Mubarak, but it is not heading down an Islamist path, as Hamas had hoped.

“They are in trouble, and so they reached out,” one Abbas aide said of Hamas.

Hamas figures presented a different picture of what led to the accord. They focused on Mr. Abbas’s frustrations with Israel and the United States in failed peace efforts and said that Fatah was therefore heading more in the direction of Hamas.

“There are no negotiations now, so let’s not speak about illusions that may or may not happen,” Taher al-Nounou, a Hamas spokesman, said when told of Mr. Abbas’s comments. “The Israeli government has nothing to offer to the Palestinians. It even refused to freeze settlements.” But he said that Hamas would abide by any P.L.O. negotiations and that it expected the P.L.O. to be reconfigured after elections in a year.

Mahmoud Zahar, a Hamas leader who was in Cairo for the Egyptian-brokered Palestinian negotiations, said he saw no place for peace talks with Israel under the new arrangement.

“Our program does not include negotiations with Israel or recognizing it,” Mr. Zahar told Reuters. “It will not be possible for the interim national government to participate or bet on or work on the peace process with Israel.”

Ahmed Youssef, a former deputy foreign minister of Hamas who now serves as a consultant to it in Gaza, said by telephone that Palestinians were “really disappointed in the Obama administration” and what they had originally been led to believe was a new American vision for the region. He added that the warmth of the new leadership in Egypt allowed it to place its confidence in its good offices.

In Gaza, people received the news of the accord with a mixture of skepticism and cautious optimism.

“I’m happy for the reconciliation but unhappy about the reasons that led to this agreement,” said Khalil Ghabin, 48, a grocer. “The changes in the region forced them to reconcile, and they did this because they were afraid that the flames of change would burn them. If there was no upheaval, they would not have agreed.”

Israeli officials reacted with horror to the prospect of a Palestinian government involving Hamas, saying it could have no place in peace talks, a stand supported by the United States.

Defense Minister Ehud Barak said in a statement: “Hamas is a murderous terrorist organization that fires rockets at citizens and recently fired an antitank missile at a school bus. It is an organization with which we have nothing to discuss, and therefore we have nothing to talk about with it.”

He said earlier in a radio interview that Israel would talk to a Palestinian unity government only if Hamas accepted internationally agreed conditions for its legitimacy, including recognizing Israel, accepting previously signed Israeli-Palestinian agreements and abandoning terrorism. Hamas officials have repeatedly rejected all three conditions.

Israelis on the left said they believed the Fatah-Hamas agreement could prove useful to achieving a two-state solution.

“I have always felt that divisions within Palestinian politics were not good for peace and see this as a step forward,” said Tamar Hermann, who leads public opinion polling on peace questions at the Israel Democracy Institute and was at the Abbas lunch on Thursday.

Efraim Halevy, a former chief of Mossad, Israel’s intelligence agency, said in a telephone interview that he had always believed that “there will be no serious progress in the Israeli-Palestinian conflict without some way of including Hamas in the process so as to transform them from being part of problem to being part of the solution.”

But this is a minority view in Israel.

In his lunch with the visiting Israelis, Mr. Abbas was asked to provide details about the accord. He said that too much needed to be worked out to say anything now. He said he was taken by surprise on Wednesday to learn that Hamas had agreed to sign and asked for time to work things out.

Asked about the future of Prime Minister Salam Fayyad, a favorite of Western powers, he said it remained undecided. But he sought to assure the Israelis that he would never allow Hamas militias to take up positions in the West Bank. Some Israeli officials have said they feared that Hamas could take over the area.