CAIRO — Rival Palestinian factions reached a key agreement in Cairo Thursday to admit the Hamas militant group into the Palestine Liberation Organization — the umbrella organization of the Palestinian independence movement that has overseen two decades of on-and-off peace talks with Israel, both sides said.

The deal marked a significant step toward unifying the long-divided Palestinian leadership and cleared the way for Hamas, which currently controls the Gaza Strip, to promote itself as the leader of all Palestinians. Hamas has historically opposed the PLO’s peace efforts with Israel, and having a strong voice in the group would further complicate the already troubled Mideast diplomatic process.

“The reconciliation has taken off. It might take time, but we have started,” said Azzam al-Ahmed, a top Fatah negotiator, after the talks in Cairo.

Under the agreement, Hamas’ supreme leader, Khaled Mashaal, joined a committee that will prepare for elections of the PLO’s parliament in exile. He will serve alongside Palestinian President Mahmoud Abbas, leader of the rival Fatah movement.

The Fatah-dominated PLO has never held a popular election in its nearly 50 years. Instead, members of its exiled parliament were chosen by individual parties and labor unions, or appointed by Palestinian leaders. The election would clear the way for Hamas to become a full member of the body.

Any PLO election is likely years away because of logistics alone. The PLO represents all Palestinians, so the vote would have to include people spread throughout the world, including residents of refugee camps in Jordan, Syria and Lebanon. In addition, political battles are likely to hinder the process.

Still, Thursday’s deal marked an important step toward reconciliation between Fatah and Hamas, whose long-stranding rivalry boiled over into civil war when Hamas overran the Gaza Strip in 2007. Since then, Hamas has ruled Gaza, while Abbas governed from the West Bank.

Both Hamas and Fatah officials said the reconciliation efforts are a result of the Arab Spring protests that have shaken up the Middle East.

Hamas is feeling emboldened by the strong showing by Islamic parties in elections in Tunisia, Morocco and Egypt. “The Arab awakening is shaping the entire region,” said Mohammed Nasr, a senior Hamas official.

Abbas also met in Cairo Thursday with Egyptian leaders.

With peace talks with Israel stalled, Abbas is eager to reunify the divided Palestinian territories.

The sides have tentatively agreed to hold separate elections next year in the West Bank and Gaza to choose a new government for the territories where Abbas hopes to establish an independent state.

On Thursday, Abbas issued a presidential decree naming a committee to oversee preparations for the local elections. The nine-member committee is headed by political independent Hanna Nasr.

Hamas favors a Palestinian state that includes Israel’s territory but has indicated willingness to accept a state in the West Bank and Gaza as a first step.

Israel objects to any Palestinian government that includes Hamas, a group that is committed to Israel’s destruction and has killed hundreds of Israelis in suicide bombings, rocket strikes and other attacks.

Hamas has sent some signals that it might be willing to reach some sort of accommodation with Israel. The group has largely adhered to a cease-fire with Israel since a brief war three years ago, and Mashaal has said he would not stand in the way if Abbas decides to resume negotiations with Israel.

Israeli government spokesman Mark Regev rejected any suggestions that Hamas is becoming more moderate. He noted that the group reiterated its calls for Israel’s destruction at its anniversary celebrations early this month.

“No one in the international community should have illusions as to Hamas,” Regev said. “This is a movement that is terrorist to the core. When Abu Mazen walks toward Hamas, he’s walking away from peace,” he said, using Abbas’ widely known nickname.