As Palestinians begin thinking about how to rebuild the bombarded Gaza Strip, the biggest hurdle quickly became apparent: Who will be in charge?

European countries, oil-rich Arab kingdoms and the United Nations have all pledged money or aid since Israel declared a cease-fire Sunday in the military offensive it launched Dec. 27. But none of the donors wants to deal with Hamas, the Islamist movement that still controls Gaza but is considered a terrorist organization by Israel, the European Union and the United States.

Israeli Prime Minister Ehud Olmert repeated his government's stance Tuesday that it would block the delivery of reconstruction aid and building materials to Gaza if they benefit Hamas.

Israel's preferred partner is the Palestinian Authority, headed by President Mahmoud Abbas and dominated by his secular Fatah party. Although the Palestinian Authority governs in the West Bank, it was forcibly tossed out of Gaza by Hamas in June 2007.

Abbas's influence has waned since then, even within Fatah. Many Palestinians are angry that he didn't object more loudly to Israel's invasion of Gaza. Instead, the Palestinian Authority's security services effectively bottled up West Bank protests against Israel's actions.

"I don't know what he's thinking," said Qaddura Faris, a Fatah leader based in Ramallah. Abbas, he said, should have cut off negotiations with Israel and led protests in the West Bank against the war. "It seems he doesn't understand the Palestinian mentality."

The task of rebuilding Gaza became clearer Tuesday as the Israeli military withdrew most of its troops from the impoverished strip of land along the Mediterranean Sea. Palestinian and U.N. surveyors estimated that more than 4,000 buildings were demolished during the fighting and that it would cost upward of $2 billion to repair the damage.

U.N. Secretary General Ban Ki-moon visited Gaza City on Tuesday and compared the area to a disaster zone. "I have seen only a fraction of the destruction," he said at a news conference. "This is shocking and alarming."

Ban criticized Israel for using "excessive" force and rebuked Hamas for firing rockets from Gaza into civilian areas in southern Israel. But he also urged Hamas to mend fences with Fatah and give Palestinians a united voice.

"I appeal to Fatah, Hamas, to all Palestinian factions, to reunite within the framework of the legitimate Palestinian Authority," Ban said.

Meanwhile, Hamas organized rallies to celebrate what it portrayed as a triumph over Israel, merely by surviving 22 days of pounding by its enemy's vastly superior military force. In Beit Lahiya, north of Gaza City, several hundred people marched behind Hamas flags and bullhorns.

"For us, this was a victory," Mohammed Abu Awad, 24, a university student, told the Associated Press.

Other residents of Gaza, however, said that the war had damaged Hamas's standing.

Nidal Muhammad, 41, a taxi driver from Gaza City, said he hoped that the Palestinian Authority would soon be restored to power in the territory. "I want the Hamas government to end. I want them to go to hell. I want the Palestinian Authority back," he said. "If Hamas serves as the resistance, this is good, but I don't want them to control everything. We want a good life for our families like before."

Hamas leaders have been cool toward the idea of a reconciliation with Fatah. They note that their movement won Palestinian elections in 2006 that were generally seen as legitimate by international observers.

They also question Abbas's continuing to serve as president; his four-year term expired Jan. 9. Abbas and his Fatah supporters say he is entitled to stay in office until Palestinian legislative elections are held in January 2010.

Fatah leaders in the West Bank said they doubted that Hamas would heed international calls to reconcile with other Palestinian factions. But they also said there was no other choice if the Palestinian push for statehood was to survive.

"If in fact, after all of this disaster, what emerges is a Gaza Strip that is viewed as a separate entity, what will be left of our national project?" Salam Fayyad, the prime minister of the Palestinian Authority, told a gathering of foreign reporters in Ramallah on Monday. "The issue goes to the core of who we are as a people and the nature of the struggle."

Fayyad said Hamas, Fatah and other Palestinian groups needed to form a national unity government that would control Gaza and the West Bank. He acknowledged that the task would be difficult but added, "Expecting it to be difficult will only make it more difficult."

He also dismissed complaints that Abbas had shown weak leadership or that Fatah had lost popular support by not being more critical of Israel.

"This is not the time to settle scores," he said. "Everyone talks about winners and losers, but we are all losers."

Egypt in particular has pushed for a rapprochement between Fatah and Hamas. In mediating talks that sought to bring an end to the fighting in Gaza, Egyptian President Hosni Mubarak said a lasting truce would be contingent on letting the Palestinian Authority back into Gaza.

Egypt has its own motives, however. Mubarak sees Hamas, a fundamentalist religious movement with links to Egypt's Muslim Brotherhood, a banned-but-tolerated opposition group, as a threat to his secular, autocratic government. And Fatah leaders and independent analysts said they were skeptical that Hamas would cede control in Gaza anytime soon.

"This is not on the agenda of Hamas for the time being," said Qais Abdul-Karim, a Palestinian Legislative Council member who belongs neither to Fatah nor Hamas.

Abdul-Karim said Abbas and Fatah faced political challenges that were just as serious. He said Abbas had nothing to show for his promises that negotiations with Israel would lead to an independent Palestinian state. Living conditions had not improved for most residents in the West Bank and people were losing faith in Abbas, he added.

"There's a lot of discontent," he said. "It's a big impediment to his stature among the Palestinian population."

Yezid Sayigh, a professor of war studies at King's College in London who served as a Palestinian negotiator in peace talks with Israel in the 1990s, said Israel and the Bush administration had failed to give much tangible support to Abbas.

But he said that Abbas and Fatah primarily had themselves to blame for not cracking down on corruption or demonstrating competence in office.

"Hamas won in Gaza because Fatah was unable to deliver either on economic progress or on the peace front," he said of the 2006 elections there. "Fatah has been incapable of repairing or reforming themselves in the West Bank since then. I don't see how it will happen in Gaza if they are let back in."

Special correspondent Reyham Abdel Kareem in Gaza City contributed to this report.