Hamas politburo member Mohammed Nazzal told AFP that he expected an agreement "within three days at the latest". "I am optimistic about that," he said.
Abbas adviser Nabil Amr gave an even tighter timeline, telling AFP he was "highly optimistic" that a deal would be reached "within 48 hours."
King Abdullah of Saudi Arabia invited the Palestinian leaders to Mecca in a bid to draw a line under the deadliest factional violence in the Palestinian territories in years.
Saudi Foreign Minister Prince Saud al-Faisal held separate meetings with Fatah and Hamas delegations in a bid to speed up agreement, a Palestinian participant said.
Mashaal, who lives in exile in Damascus and is one of Israel's most wanted men, urged both sides' fighters to exercise restraint.
Mashaal said it was vital that the major powers respect any deal reached between the two sides.
"The international community must respect our accord, recognize our Palestinian reality and deal with it seriously," he said.
Abbas's aide said agreement had already been reached on some of the details of power-sharing that had blocked a deal in the past.Palestinian President Mahmoud Abbas and Hamas politburo chief Khaled Mashaal held crunch reconciliation talks Wednesday after vowing not to leave Mecca without a deal to end their deadly power struggle.