TEL AVIV—Israel and Gaza militants on Sunday traded cross-border strikes for a third day that killed a 12-year-old Palestinian child and blasted an empty Israeli school, as both sides said they were seeking to contain the worst violence in six months.

Militants from the Gaza Strip have fired at least 130 missiles in retaliation for Israel's assassination Friday of a Palestinian militant leader it accused of plotting an attack from Egypt's Sinai desert into Israel. Those rockets have seriously injured one person and forced the cancellation of school on Sunday and Monday for hundreds of thousands of Israeli children, said the Israeli army.

Israeli air attacks have killed at least 18 Gazans during the current crisis. Israel's army said that most of them were militants, but Gazan officials said that at least two were civilians, including the boy.

The violence highlights how the turmoil in Egypt has destabilized the border region between Israel, Gaza, and Sinai. Israel sees the vast Sinai Peninsula as an emerging threat because it has become a haven for militants amid weakening central authority.

Despite the toll, Hamas Prime Minister Ismail Haniyeh said his militant group, which controls Gaza, was in talks with armed groups in the coastal strip. He said the government of Egypt was working "hour by hour" to reach a cease fire.