Israeli aircraft and ground forces struck Gaza on Friday, killing four Hamas militants and five civilians in a surge of fighting sparked by a Palestinian rocket attack on an Israeli school bus the day before.

Israel and Gaza's Hamas rulers seemed on the brink of another round of intense violence, just a little over two years after a three-week war in which persistent rocket fire from Gaza triggered a devastating Israeli military offensive in the territory.

Israeli Prime Minister Benjamin Netanyahu said the attack on the school bus "crossed a line" and warned that "whoever tries to harm and murder children will pay with their life."

In Thursday's attack, Gaza militants hit an Israeli school bus near the border with a guided anti-tank missile, injuring the driver and badly wounding a 16-year-old boy. Most of the schoolchildren on the bus had gotten off shortly before the attack. Hamas, which had largely held its fire since Israel's last major offensive, claimed responsibility.

By Friday evening, Israel's ongoing retaliation had killed 14 Gazans _ seven militants, a policeman and six civilians _ and wounded 65. The dead Friday included three civilians killed by Israeli tank fire and three militants killed in an air strike, both near the southern Gaza town of Khan Younis. Another militant was killed in an airstrike in northern Gaza.

An Israeli tank shell struck a cemetery east of Gaza City, killing two civilians, including an 11-year-old boy, said Palestinian health official Adham Abu Salmiya.

Gaza militants fired 16 rockets and mortars at southern Israel on Friday, causing no injuries, the Israeli military said.

For the second day in a row, Israel's cutting-edge Iron Dome missile defense system successfully intercepted a Palestinian rocket aimed at an Israeli city, the military said. Reinforced rooms and early warning systems also have helped keep Israeli casualties low.

Studies at some Israeli schools near Gaza were canceled Friday because of concerns for the students' safety.

It is unclear if Hamas was trying to provoke a new conflagration, if it was not fully in control of all of its fighters, or if it believes Israel would pull back before invading Gaza again. Israel was condemned internationally after the last incursion.

Hamas said the rocket attack was in retaliation for the killing of three fighters in an airstrike earlier in the week. At around midnight Thursday, with Gaza rocked by explosions, the organization announced a cease-fire, though it later claimed responsibility for some of Friday's strikes. A small PLO faction, the Popular Front for the Liberation of Palestine, also said it fired at Israel on Friday.

The Israeli strikes continued, hitting Hamas facilities and smuggling tunnels, after Hamas announced its cease-fire. Electricity lines and transformers were damaged, causing blackouts in some parts of the territory, according to Jamal Dardsawi, a spokesman for Gaza's Electric Distribution Company.

U.N. Secretary-General Ban Ki-moon on Friday condemned the bus attack and expressed concern over civilian casualties in Israel's strikes. He called for "de-escalation and calm to prevent any further bloodshed."

Thousands of rockets from Gaza have hit Israeli towns and cities since 2001. Israel's attempts to stop the rockets have included military incursions and covert operations abroad aimed at disrupting Hamas' efforts to procure arms.

In February, a Palestinian engineer was seized from a sleeper train in Ukraine and showed up several days later in Israel, where he has been charged with masterminding Hamas' rocket program. Last year a Hamas operative was assassinated in Dubai, and Israeli agents are widely assumed to have been responsible. Israel identified the man as a Hamas agent responsible for obtaining weaponry from Iran.

This week, Sudan accused Israel of being behind an explosion that killed two in Port Sudan. The blast was thought to be linked to arms smuggling to Gaza. Israel would not comment.

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Ibrahim Barzak contributed reporting from Gaza City, Gaza Strip.