Ehud Olmert, the Israeli prime minister, has also said that he will hold trilateral talks with Condoleeza Rice, the US secretary of state, and Abbas on February 19.
The al-Qassam Brigades man was shot dead by unknown men who opened fire on his car, a security source said. Three other people were wounded in the incident.
An officer with the Executive Force, linked to the Hamas-controlled Palestinian interior ministry, confirmed the death without providing details of the victim.
Hamas officials accused gunmen from Fatah of opening fire at the vehicle from a checkpoint near a heavily-guarded security complex and the residence of Mohammed Dahlan, a senior Fatah member, who is in Saudi Arabia for the talks.
A local clan claimed responsibility for the attack and said the Hamas commander was responsible for the killing of two family members in December.
Abdel-Hakim Awad, a Fatah spokesman, denied Fatah was involved in the shooting.
|Haniya, left, and Meshaal are hopeful |
they can reach an agreement [Reuters]
Both sides have said they are hopeful that an agreement on a unity government will be reached at the Mecca summit.
But as Ismael Haniya, the Palestinian prime minister, left for the talks, Hamas and Fatah security officials traded fire at the Gaza-Egypt crossing.
"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.
Before Abbas 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."
King Abdullah said he wanted the meeting to lead to a resolution of the inter-Palestinian fighting.
"I hope that the brothers will not leave the holy place without an agreement ... and that they will swear ... to stop this bloodshed," he was quoted by the official news agency SPA as saying.
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.
The talks are seen as a last-ditch effort to reach a power-sharing agreement and end the factional fighting that has killed 64 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.
"It is pitiful to witness the bloody scenes in Gaza between the brothers"
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.
Elsewhere on Tuesday, Ehud Olmert announced he expected a three-way summit with Abbas and Condoleezza Rice to take place on February 19.
"In a few days, on the 19th, I hope to have a trilateral meeting with secretary of state Condoleezza Rice and chairman of the Palestinian Authority Abu Mazen [Abbas]," he said in a speech to American Jewish leaders.
"I hope that by then we will not find that Abu Mazen has partnered with Hamas in a way that is in contradiction of the principles of the quartet and the international community."
Olmert did not specify where the summit would be held.
Source: Al Jazeera and agencies