Jerusalem ? Israel expressed regret Thursday for the deaths of four Palestinian civilians in army shelling a bloody incident that jeopardized shaky steps toward reducing tensions in Gaza.

The need for a peace breakthrough got reinforcement from a U.N. report that said Palestinians were suffering from ever-worsening economic deprivation and the threat of malnutrition as a result of two years of Palestinian-Israeli fighting.

Violence continued Thursday. In Rafah, on the Gaza-Egypt border, Israeli soldiers shot and killed a 12-year-old Palestinian boy, Palestinians said. Residents said Israeli forces opened fire after children threw rocks and bottles at Israeli tanks that were tearing down structures.

The Israeli military said soldiers in Rafah found a tunnel used for smuggling arms across the border. The tunnel was in a house, the military statement said, and soldiers blew it up. During the operation, Palestinians fired rifles and threw hand grenades, firebombs and rocks at the soldiers, who returned the fire, the military said.

In the West Bank city of Nablus, Israeli forces entered the Palestinian government headquarters and set off explosives, a witness said. Residents said they heard a huge explosion all over the city, and witnesses said part of the fortress-like three-story building was destroyed, including the office of the governor, Mahmoud Aloul, who was not in the building at the time. No casualties were reported.

Aloul said there were no explosives or other weapons inside. He told The Associated Press the Israeli operation “is proof of the Israeli aggression against the Palestinian Authority and the Palestinians.”