Palestinian leaders from Fatah and Hamas are to meet in Mecca for talks aimed at ending months of factional fighting.
Both sides have said they are hopeful that an agreement on a unity government will be reached, but as the Palestinian prime minister, left for the talks, Hamas and Fatah security officials traded fire at the Gaza-Egypt crossing.
Before the Palestinian president arrived in Saudi Arabia, he said that failure could bring about a civil war, Lebanese newspaper Al-Akhbar reported.
"Failure would mean the deterioration of the internal situation and igniting civil war," Mahmoud Abbas said. "The word 'failure' is forbidden."
Khaled Meshaal, Hamas's Damascus-based political chief, and Ismail Haniya, the prime minister, were due to arrive in Mecca on Tuesday afternoon.
"We promise our people that we will do all we can and will exert every effort in order to reach a Palestinian agreement over the formation of a unity government," Haniya said before heading for the talks, called by King Abdullah of Saudi Arabia.
Meshaal and Haniya are expected to perform the minor pilgrimage at the Grand Mosque in Mecca, Islam's holiest site, before beginning talks in the evening which are expected to continue into Wednesday.
Commitment urged
King Abdullah made clear he hoped the setting would have an influence when he issued a statement to the Palestinian community in Saudi Arabia on Tuesday.
"I hope that the Palestinian brothers hear your demand, and that they will not leave the sacred land without a commitment before God to stop fighting and bloodshed," the king said.
The talks are seen as a last-ditch effort to reach a power-sharing agreement and end the factional fighting that has killed 63 Palestinians since January 25.
Negotiators have tried but failed for months to find common ground on the key issues of ties with Israel and the division of portfolios in a unity accord, in the hope of ending a boycott by the US and European Union.
Washington and Brussels consider Hamas to be a "terrorist" group and suspended direct financial aid to the Palestinians when the current Hamas-led cabinet took power in March.
Peace deals
The Hamas-led government has resisted pressure to renounce violence, recognise Israel and abide by peace deals between the Palestine Liberation Organisation (PLO) and Israel.
Azzam al-Ahmad, a senior aide to Abbas, said the talks would aim to persuade Hamas to accept the programme of the PLO which would involve an implicit Hamas recognition of Israel.
"This won't contradict the requirements for lifting the siege ... I'm sure once Hamas honours PLO agreements the Quartet will not be asking Hamas to recognise Israel any more," he said, referring to the group of Middle East peace mediators including the US, EU, United Nations and Russia.
The discussions are also expected to focus on the division of cabinet positions in the new government and how to unify the many security forces that support the two factions.
Previous efforts to end the fighting and find common political ground have resulted in short-lived ceasefires and a threat by Abbas to call new elections, a move Hamas has said would be tantamount to a coup.
Photo caption
Mahmoud Abbas (L) and Khaled Meshaal