JERUSALEM — President Mahmoud Abbas of the Palestinian Authority embraced reconciliation with the Islamist movement Hamas on Monday, agreeing to head a unity government to prepare for elections in the West Bank and Gaza.

Mahmoud Abbas, the president of the Palestinian Authority, left, Sheik Hamad bin Khalifa al-Thani, Qatar’s Emir, and Khaled Meshal, the leader of the Islamic Hamas movement, in Doha on Monday.

His move was welcomed cautiously by a broad range of Palestinians who are fed up with the brutal split at the heart of their national movement. It promised to upend Israeli-Palestinian relations, with Prime Minister Benjamin Netanyahu warning Mr. Abbas that he could have peace with Israel or unity with Hamas, but not both.

The agreement between Mr. Abbas, the leader of Fatah, and Khaled Meshal, the head of Hamas, was yet another convulsion in the Middle East involving the rise of political Islam and the challenge it poses to pro-Western forces. It put Israel, which is nervously watching the new order taking shape around it, further on edge.

“Hamas is an enemy of peace,” Mr. Netanyahu said. “It’s an Iranian-backed terror organization committed to Israel’s destruction.”

On Sunday he told his cabinet that for Israel, living in the Middle East required self-sufficiency and toughness. “In such a region,” he said, “the only thing that ensures our existence, security and prosperity is our strength.”

Mr. Abbas and Mr. Meshal announced their agreement on Monday in Doha, the capital of Qatar. Hamas has had to leave its longtime base in Damascus, the Syrian capital, because of the unrest and violence there, and Qatar appears to be seeking the role of Hamas’s new sponsor.

The two Palestinian leaders said they would announce a full government in the next week or two, along with a date for presidential and legislative elections. It was unclear what role the current prime minister, Salam Fayyad, would play in the interim government. Mr. Fayyad is admired abroad for his financial transparency, and is the reason that some countries provide aid to the Palestinian Authority — more than $1 billion annually in total. But Hamas leaders have in the past expressed their distaste for his policies.

The planned elections are unlikely to take place this spring, as promised last May when the Hamas-Fatah unity accord was first signed. Many of the details are bound to produce a struggle, and Palestinians greeted the news on Monday with relief but with skepticism, especially in Gaza.

“The Palestinian people look suspiciously at Fatah-Hamas understandings because they have been repeated dozens of times without finding their way to implementation,” said Mkhaimar Abusada, a political science professor at Gaza’s Al Azhar University.

This latest signed document may face the same fate. The rival movements have to negotiate the terms of complex power sharing and the restructuring of the Palestine Liberation Organization, from which Hamas has been excluded.

It remained unclear how some of the Hamas leaders in Gaza, who are destined to lose their jobs in the new arrangement, would react to a deal struck by Mr. Meshal, who lives in exile and recently said he would not seek a new term as head of the movement.

In Washington, the Obama administration publicly withheld judgment on the agreement, saying that American officials were still trying to determine the details of a unity government. The agreement, however, revived questions about the future of American assistance to the Palestinian Authority.

Congressional amendments forbid foreign aid going to Hamas, which the United States has designated a terrorist organization. A partnership with Mr. Abbas could lead to a cutoff. “It further jeopardizes whatever existing aid is left,” said Representative Gary L. Ackerman, a Democrat from New York.

Until now, the State Department has declined to restrict aid, including military assistance to Palestinian security forces that totaled $450 million last fiscal year. The department has argued that the prospect of a Palestinian unity government that included Hamas, first announced last year, never fully materialized. The aid has been credited by Israeli and American officials for improving security in the Palestinian territories.

If Monday’s agreement takes root, it would force the issue, putting the administration in an awkward position, especially with Congress, according to an administration official, who spoke on condition of anonymity.

The State Department’s spokeswoman, Victoria Nuland, said that Palestinian reconciliation was “an internal matter” for the Palestinians but added that the administration would expect any Palestinian government to meet basic conditions, including recognition of Israel.

Mr. Ackerman cited Mr. Meshal’s statement about unifying against the “enemy” as evidence that Hamas remained unrepentant. “It’s not conciliatory,” he said in a telephone interview. “It continues the saber rattling and the threat.”

But some analysts argued that the regional shifts of the last year and the failure of recent Palestinian-Israeli talks to reach a breakthrough were pushing Fatah and Hamas into each other’s arms. They said that Hamas would soon undergo some of the changes that Islamist movements elsewhere in the region are seen by some to be experiencing.

“The Arab awakening is witnessing the rise of a reformist political Islam in Egypt and Tunisia, and I believe we will see that Hamas is no exception,” asserted Mahdi Abdul Hadi, chairman of Palestinian Academic Society for the Study of International Affairs in Jerusalem. “Western governments are dealing with the Muslim Brotherhood in Egypt, and it is only a matter of time before they will meet with Hamas as well.”

There are senior defense officials in Israel who see a significant shift happening in Hamas as well. One, speaking recently on condition of anonymity, said, “Hamas is learning that governance is more important than terrorism.”

Mr. Netanyahu disagrees that Hamas is changing. He noted in his statement on Monday that until Hamas recognizes Israel, abandons violence and accepts previous agreements with Israel signed by the Palestinian Authority — the three conditions that the United States and the European Union demand of Hamas, which has rejected them — it remains a renegade that must be shunned.

For Mr. Netanyahu, who leads the hawkish Likud Party and a coalition Israeli government with a strong base among Jewish settlers and their supporters, Palestinian unity poses a complex set of choices.

On one hand, Mr. Netanyahu says that he seeks peace with the Palestinians, and that the formation of a joint Fatah-Hamas government would appear to deprive him of the chance to pursue what he has called a historic opportunity for peace. On the other hand, a reconciliation of the two factions would free Mr. Netanyahu of the burden of those difficult negotiations, where he comes under international pressure to yield prisoners, land and greater power to the Palestinian Authority.

He could instead turn his back on the whole endeavor, which would secure him against any political challenge from the right wing just as the possibilities of elections appear on the horizon.

Mr. Abbas has his own tortured calculations. He has been pursuing three tracks toward Palestinian statehood; all have proved problematic. The first has been his recently renewed talks with the Israelis under Jordanian auspices, which have gone poorly. The second is the track of unity with Hamas, which until Monday seemed stuck and which remains far from stable.

The third is his efforts at the United Nations, meant to obtain international backing for Palestinian sovereignty in the West Bank, Gaza and East Jerusalem; that track has proved harder than expected. Last September, Mr. Abbas was unable to get enough members of the United Nations Security Council to vote yes for recognizing Palestine as a state. He did gain membership in Unesco, a United Nations agency, but that led the United States to cut off American funds to that organization and to a pause in the Palestinian efforts in international bodies.

But Palestinian officials say that Mr. Abbas is likely to revive that path in the coming weeks, especially if the Israeli track stalls, as many expect.

An abandonment of negotiations with Israel brings with it risks, in particular that Hamas will campaign on its long-standing assertion that talks with Israel were a humiliating waste of time and that Hamas’s approach of resistance and links to the broader Islamic movement deserve the people’s votes.

In addition, Israel has a great deal of power over the Palestinian economy, and could make it suffer. Israel could also make the lives of Palestinian officials even harder than they are, by denying them travel privileges.

Qatar, a Gulf emirate that is both wealthy and diplomatically ambitious, could prove to be a crucial element in helping the Palestinians. Qatar is already spending money in Gaza to help the territory rebuild and rehabilitate from the Israeli invasion there three years ago; the emirate could both greatly increase its spending there and make up for missing aid to the Palestinian Authority.