GAZA — In a blow to Palestinian unity efforts, a meeting in Cairo planned for this week so that the leaders of the two main factions could announce a new government has been called off for lack of agreement on a prime minister, Palestinian officials said Sunday.

The meeting had been set for Tuesday with President Mahmoud Abbas of the Palestinian Authority and Khaled Meshal of Hamas. Mr. Abbas’s Fatah faction announced the cancellation in the West Bank, and Hamas officials here confirmed it.

Mr. Abbas had been hoping to forge an image of a united Palestinian front as he laid the groundwork for seeking international recognition of statehood at the United Nations in September.

Mr. Abbas has been pushing hard to keep in place Prime Minister Salam Fayyad, who is widely admired in the United States and Europe, the sources of hundreds of millions of dollars in annual aid to the Palestinian Authority, and in Israel, whose security officials cooperate with those under him in the West Bank.

But Hamas leaders despise Mr. Fayyad, considering him a stooge of Israel and the West and blaming his security authorities in the West Bank for arresting and mistreating their followers.

“Salam Fayyad is a criminal who should be put on trial,” Mahmoud Zahar, a top Hamas official, said in an interview at his home. “He has tortured our people in prison. He is not acceptable to anyone in Hamas.”

The announcement of the meeting’s cancellation was made by Azzam al-Ahmed, chief Fatah negotiator in the reconciliation talks, who said another meeting would be scheduled soon.

But other officials said the problem was unlikely to be resolved quickly. Some said that Washington had warned Mr. Abbas that going ahead with the unity government would risk American aid, which currently amounts to about $500 million a year.

The United States, like Israel, considers Hamas to be a terrorist organization, and Congressional rules on aid to the Palestinian Authority are tightly drawn to exclude any money going to Hamas-affiliated bodies.

Although the unity government would be composed of unaffiliated technocrats — Mr. Fayyad is not a member of Fatah — it would most likely be confirmed by the Palestinian Legislative Council, which has been dominated by Hamas since that group won elections in 2006.

Israel has also said that any unity deal would sink the possibility of returning to negotiations. Hamas and Fatah disagree on the negotiations with Israel as well, with Hamas opposing them.

The surprise deal between Fatah and Hamas, brokered by Egypt, was signed in early May after four years in which the two groups barely spoke. It called for the formation of a national unity government and elections a year later.

The desire for unity is widely shared by the populations in both Hamas-ruled Gaza and Fatah’s West Bank, but the efforts to agree on a government and make it function face enormous challenges not easily overcome. Still, leaders in both places say unity is vital.

“This reconciliation is a must,” said Muhammad Shtayeh, a member of the Fatah Central Committee, speaking by telephone from Ramallah in the West Bank. “We cannot afford to lose this. It is our duty, our responsibility to make it happen.

“For me, the issue is not individuals but the political program. We have to sit down and agree on a political agenda, but that is not there yet either.”