U.N. Secretary-General Ban Ki-moon on Monday urged Arab leaders to join together in backing Palestinian President Mahmoud Abbas in his efforts to reunite the war-ravaged Gaza Strip with the West Bank.

Speaking a day after Israel and the militant Palestinian group Hamas announced separate ceasefires, Ban also said that Arab unity was crucial if the three-week Gaza conflict was not to be repeated in the future.

"The Palestinians themselves must face the challenge of reconciliation, and work to achieve a unified government under the leadership of President Abbas," Ban told an Arab League summit expected to approve $2 billion in aid to rebuild Gaza.

"I call on all Arab leaders to unite and support this endeavour. We cannot rebuild Gaza without Palestinian unity."

The Gaza conflict has divided Arab countries, as recent meetings of Gulf states have shown.

Egyptian President Hosni Mubarak, who negotiated with both Hamas and the Israelis to get a ceasefire, called for uniting all Palestinian factions in his speech at the summit. Syrian President Bashar al-Assad, whom Israel accuses of financing Hamas, voiced support for "Palestinian resistance."

Hamas militants wrested control of the Gaza Strip from Abbas's Palestinian Authority in 2007. Abbas, who still controls the West Bank, is seen as weak and ineffectual by leaders of some Arab countries like Syria.

Ban has been touring the Middle East for a week urging Israeli leaders and Arab governments to do everything in their power to end the fighting in Gaza and prevent the humanitarian crisis for the enclave's 1.5 million people from worsening.

He told reporters during the flight to Kuwait that if Arab states remain divided on Abbas and Palestinian unity, there was "no guarantee this (the Gaza conflict) will not happen again."

In his speech in Kuwait, Ban reiterated that Israel must reopen border crossings with Gaza, allow humanitarian aid in and withdraw from the Gaza Strip. Likewise, he urged Hamas to stop firing rockets at southern Israel.

But a permanent solution, he said, would require a return to the stalled Middle East peace process.

"A true end to violence, and lasting security for both Palestinians and Israelis, will only come through a just and comprehensive settlement to the Arab-Israeli conflict," he said. "The (Israeli) occupation that began in 1967 must end."