GAZA CITY, Gaza Strip — The official spoke on condition of anonymity because of the sensitivity of the indirect negotiations.
U.N. Security General Ban Ki-moon also arrived in Cairo to appeal for an end to violence.
The rising toll was likely to intensify pressure on Israel to end the fighting. Hundreds of civilian casualties in an Israeli offensive in Gaza four years ago led to fierce international condemnation of Israel.
But Mashaal said Gazans were prepared to keep fighting.
"Gaza's demand is not a halt to war. Its demand is for its legitimate rights," including a stop to Israeli attacks, assassinations and a lifting of the blockade, Mashaal said.
Israel has been jittery that a second front along its northern border could be opened, either by militants in Lebanon or from spillover from the Syrian civil war.
Lebanese military experts dismantled two Katyusha rockets Monday that were equipped with timers and ready to fire at Israel, a senior Lebanese security official said, speaking on condition of anonymity in line with military regulations.