GAZA CITY -- Palestinian militants fired more than 30 rockets at southern Israel and Israeli warplanes killed five militants in the Gaza Strip Saturday.

The escalating series of strikes and counterstrikes has killed 18 Gazans since it began on Thursday, following a Hamas missile attack against an Israeli school bus.

Israeli officials said the airstrikes would continue as long as the rocket attacks persist. Hamas officials said they were trying to restore quiet, but that they were prepared to fight.

"Hamas will not stand idle in front of this escalation," Hamas spokesman Sami Abu Zuhri said. "We have exercised a limited response so far, but we warn the occupation against continuing with their crimes."

After two years of relative calm, the situation began to deteriorate nearly three weeks ago with rocket fire toward Israel and a retaliation that killed Gazans.

It escalated dramatically last weekend when an airstrike killed three Palestinian militants who Israel said were plotting to carry out a cross-border kidnapping.

On Thursday, Hamas militants fired an anti-tank missile at an Israeli school bus, wounding two people on board, including a teenage boy.

Israel's prime minister, Benjamin Netanyahu, said that act crossed a line and he vowed a harsh response.

Gaza militants fired more than 30 Grad-style Katyusha rockets and more than 10 mortar shells into Israel Saturday in what Israeli police said was the biggest single-day bombardment since the Gaza war.

Several missiles fell in uninhabited areas near the southern Israeli cities of Ashkelon and Beersheba, while seven were intercepted by Israel's new missile shield, Iron Dome. There were no reports of injuries.

Iron Dome, activated earlier this week, is the first system in the world able to knock down short-range rockets. The homegrown system could potentially alter the balance of power if it proves able to consistently stop a large number of rockets.

Thousands in Gaza attended funerals for the militants slain Saturday, among them a Hamas commander in the southern city of Rafah.

In Gaza City, crowds of onlookers, including groups of children, watched as bodies wrapped in green Hamas flags were carried through the streets. Militants fired in the air and chanted, "The blood of our martyrs will not be spilled in vain."

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