At least four Hamas fighters were reported killed in Saturday's Israeli bombardment. In all, 18 Gazans have been killed and more than 65 wounded in Israeli strikes since Hamas attacked an Israeli school bus with an anti-tank rocket Thursday. The bus attack seriously wounded a 16-year-old boy and injured the driver.

Israeli Prime Minister Benjamin Netanyahu has warned that the bus attack crossed a red line and that Israel would strike back hard — in an apparent attempt to restore the deterrence that had been created by the Gaza war in the winter of 200-2009.

Hamas leaders, meanwhile, said Saturday they wouldn’t hold back if Israeli strikes continue. “Hamas will not stand idle in front of this escalation,” Hamas spokesman Sami Abu Zuhri told a news conference. “We have exercised a limited response so far, but we warn the occupation against continuing with their crimes.”

On Saturday, Gaza militants fired 17 longer-range Grad missiles and seven mortar shells into Israel in the biggest single-day attack since the Gaza war, Israeli police said.

Several missiles fell in open land near the southern Israeli cities of Ashkelon and Beersheba, while seven were intercepted by Israel’s new missile shield, Iron Dome. A mortar shell hit a local government building in a small community, causing some damage. There were no reports of injuries.

Iron Dome, a new missile shield first activated earlier this week, is the first system worldwide to knock down missiles at such altitude and trajectory.

In Gaza, meanwhile, an Israeli airstrike struck a car near Rafah in southern Gaza early Saturday, killing three Hamas militants. Hamas said one of its top commanders, 29-year-old Tayser Abu Snima, and two of his assistants were killed. The Israeli military said Abu Snima had been involved in attacks on Israeli soldiers and civilians.

Later, Hamas said a tank shell killed another militant near the Jabalya neighborhood in Gaza. The Israeli military said it was not aware of a strike involving a tank shell.

Overall, the Palestinian death toll since Thursday includes 11 militants, a Hamas policeman and six civilians.

Thousands attended Saturday’s funerals. In Gaza City, crowds of onlookers, including groups of children, watched as bodies wrapped in green Hamas flags were carried through the streets. During the funeral, militants fired in the air and chanted, “The blood of our martyrs will not be spilled in vain.” The latest round of fighting seems to end a period of relative restraint by both sides since the Gaza war.

Israel had launched that three-week offensive to put an end to persistent rocket attacks from Gaza on its southern border towns. Some 1,400 Gazans were killed in the war, including hundreds of civilians, and Israel drew worldwide condemnation for what some said was excessive force against one of the world’s most crowded territories.

However, Hamas largely held its fire after the war, while Israel largely refrained from carrying out targeted killings of Gaza militants, once a common practice.

Thursday’s bus attack marked a turning point for Hamas.

It remains unclear whether it was carried out by rogue militants going against the movement’s orders or was authorized by Hamas leaders. However, those behind the attack must have taken a harsh Israeli response into account. Hamas may also be trying to test whether, in the wake of the Gaza war, Israel will be more subdued in its retaliation.

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.