The U.N. envoy to the Middle East has expressed concern over what he described as Israel's "lack of progress" in apprehending suspected Jewish extremists responsible for a July arson attack that killed a Palestinian couple and their toddler.

Nickolay Mladenov called for "justice" in a statement released after 27-year-old Rihan Dawabsheh died early Monday of her wounds from the attack. Her husband Saad died earlier while their 18-month-old son Ali was burned to death in the attack.

The couple's 4-year-old son Ahmad is still undergoing treatment at an Israeli hospital.

The attack drew widespread condemnation and Israel pledged to get tougher on Jewish extremists in the West Bank suspected in attacks on Palestinians and their property.

Israeli authorities have not yet arrested or identified suspects in the July arson attack.