Israeli strikes in the Gaza Strip on Friday and early Saturday killed 13 Palestinians, including a woman and her daughter, medical officials said, in continuing exchanges of fire following a missile attack on an Israeli school bus that critically wounded a teenager.

Israeli aircraft and tanks fired at targets in the Palestinian enclave, and militants launched mortar rounds and rockets at Israel, causing no casualties. Seventeen Palestinians, both militants and civilians, have been killed and dozens wounded since Thursday’s bus attack, in the worse flare-up of violence since an Israeli offensive in 2009 against the militant Islamic group Hamas, which controls Gaza.

The Israeli army said it had killed “approximately 15 terrorists” and wounded 50 others.   

U.N. Secretary General Ban Ki-moon urged an “immediate end” to rocket fire at Israel and “maximum restraint” by Israel, voicing concern at the reports of civilian casualties.

The Izzedine al-Qassam Brigades, the armed wing of the Hamas, said it lost two senior commanders in an airstrike in the southern town of Rafah, two men east of Khan Younis and three more in two separate strikes in the northern Gaza Strip.

An Israeli drone hit a house east of Khan Younis, killing a 45-year-old woman and her 21-year-old daughter, according to a spokesman for Gaza’s emergency services. He said a 55-year-old farmer was also killed in a separate attack in the same area.

The Israeli army said that it had responded to mortar and rocket fire from a neighborhood in the Khan Younis area and that two launching squads were targeted and hit. A military spokesman acknowledged that “uninvolved civilians” appeared to be among the casualties but accused Hamas of using them as human shields by operating inside residential areas.

A militant and an 11-year-old boy were killed and 10 other people were injured, most of them children, by Israeli shelling in Shujaiya, east of Gaza City, medical officials and the Islamic Jihad group said.

The army said it had struck from the air at a squad that had fired mortars at Israel and that it was not aware of civilian casualties.

The body of another Palestinian, apparently killed in earlier fighting, was recovered in Khan Younis.

The army said that about 70 mortar rounds and rockets were fired at Israel on Friday and Saturday, including three Grad rockets aimed at the southern coastal city of Ashkelon and two aimed at the southern city of Beersheba that were intercepted by the military’s new Iron Dome missile defense system. The squad that fired the Grad rockets at Ashkelon was targeted from the air and killed, the army said.

A top Israeli general called Thursday’s antitank missile attack on the school bus, for which Hamas asserted responsibility, a “new threat” by the group.

“Red lines were crossed,” said Maj. Gen. Tal Russo, the chief of the military’s southern command.

Special correspondent Islam Abdel Kareem in Gaza contributed to this report.