EL-ARISH, Egypt - A group of armed men shot dead a tribal leader and his son on Monday in Egypt's Sinai peninsula on the border with Israel, a security source said, as violence escalated on the sixth day of a military crackdown on Islamists in the area."Tribal leader Khalaf Al-Menahy and his son were shot dead by militants on their way back from a conference organized by tribal leaders to denounce militancy," said the security source in Sinai. The attack occurred during a security sweep that began on Wednesday after the killing of 16 Egyptian border guards on Aug. 5, which Egypt blamed on militants.The military operation is the biggest in the region since Egypt's 1973 war with Israel.Lawlessness has been growing in Sinai, a region awash with guns and bristling with resentment against Cairo, since the overthrow of Hosni Mubarak in an uprising last year. Parts of northern Sinai have been controlled by Bedouin tribes since police deserted the area during the uprising.Another source close to Islamists in Sinai said hundreds of them had organized a secret meeting on Sunday night to discuss their response to the killing of five suspected terrorists by Egyptian soldiers earlier on Sunday."They agreed that the reaction will be harsh," the source said.The military crackdown in the Sinai peninsula is seen as an early test for Egyptian President Mohamed Morsy - a moderate Islamist elected in June- to prove he can rein in the militants whose activity near the border worries both Egyptians and Israel.Morsy dismissed two top generals on Sunday, quashing a military order that had ruled the transition period after Mubarak and curbed Morsy's presidential powers. Last week, he fired North Sinai's governor and Egypt's intelligence chief.Morsy's critics say the Islamist leader risks being seen as soft on jihadists because he is from the Muslim Brotherhood, a political movement that has ties to the Hamas government in Gaza.