GAZA CITY, Gaza Strip -- Palestinian militants fired more than 30 rockets at southern Israel on Saturday and Israeli warplanes killed five militants in the Gaza Strip in the most intense fighting since Israel's 2008-2009 offensive in the Hamas-ruled territory.

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. The continuing clashes are increasing the probability of a full-scale military confrontation.

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

Hamas says Israel has used excessive force in its retaliation, which has killed six civilians and wounded more than 65 people.

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, but damage was caused.

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. Israeli officials stress it is still largely unproven.

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

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."

Hamas, a virulently anti-Israel militant group backed by Iran, has controlled Gaza since seizing the territory in a brief civil war against the rival Fatah faction in 2007. Late the following year, Israel invaded the territory to stop years of persistent rocket fire.

During the three-week operation, Israeli forces killed some 1,400 people, including hundreds of civilians, while 13 Israelis were also killed. After suffering heavy losses, Hamas has largely honored an informal truce since then.

Israel believes Hamas has recovered from the war, rebuilding its forces and restocking its arsenal with deadlier weapons. Israel believes Hamas now has rockets capable of striking as far as Tel Aviv, some 50 miles (80 kilometers) north of Gaza, as well as anti-tank and anti-aircraft missiles it did not possess during the war.

Israel's battle against Hamas has stretched well beyond Gaza. In recent weeks alone, the Israeli navy intercepted a cargo ship that was carrying arms allegedly bound for Hamas. It also mysteriously captured a Gaza engineer, who was last seen in Ukraine, and charged him with being a rocket mastermind.

Last week, Sudan accused Israel of carrying out a covert airstrike that killed two people. Israel has not commented, but officials have repeatedly said they believe Sudan is a transit point for illicit arms headed to Gaza.

Both sides may be using the current flare-up to test each other's capabilities in the wake of the 2008-2009 war. Israel would want to see what weapons Hamas really has obtained, while Hamas has an interest in seeing whether the much-touted Iron Dome system really works.

Still, both sides were signaling they hope to restore the calm. Israeli officials said the rocket fire, and particularly Thursday's bus attack, required a strong response. The officials, speaking on condition of anonymity because they were discussing strategic deliberations, said the airstrikes were aimed at deterring Hamas and would continue until the rocket fire stops.

Israeli military analyst Reuven Pedatzur said Israel has no interest in a large-scale assault on Gaza, but that if casualties mounted it may be forced to act.

"The sad irony is that it all depends on a single rocket. If it lands and kills three kids, there will be an operation. If not, there won't be," he said.

Ihab Ghussein, spokesman for the Hamas Interior Ministry, which oversees the group's security forces, said Hamas has been in touch with various militant factions in Gaza, as well as with regional governments in hopes of restoring the calm.

"We have already stated our position that we want to maintain calm and stability in the Gaza Strip," he said. "But the occupation forces insist on destroying all the understandings and carrying out a bloodbath in Gaza."

However, a spokesman for Hamas' military wing, Abu Obeida, said that calm was not an option under the current circumstances.

Hamas officials said they are ready to fight and believe conditions are far better now than two years ago. The group said it had fired 68 rockets and mortar shells over the past two days.

Hamas also feels its international position is much stronger now following the revolution in neighboring Egypt. Former President Hosni Mubarak, ousted in February by a popular uprising, was a staunch ally of Israel in its battle against Hamas. As Egypt prepares for elections, potential leaders have signaled they will take a more lenient approach toward Hamas.