GAZA (Reuters) - Fighting flared in Gaza on Thursday after a Palestinian anti-tank missile hit an Israeli school bus, wounding two, and Israeli forces retaliated with planes and artillery, killing five Palestinians.

Israeli soldiers walk past a bus that was damaged by an anti-tank missile fired from the Gaza Strip into southern Israel on the road between kibbutz Nahal Oz and kibbutz Sa'ad April 7, 2011. . REUTERS/Baz Ratner

Palestinian medics said at least 30 people were injured in three hours of attacks by Israeli forces. Firing tapered off after nightfall.

A 50-year old Palestinian in east Gaza was killed by shelling in the afternoon and four others were killed by air attacks in the south near the border with Egypt.

A teenage Israel boy on the bus was seriously wounded and its driver was injured and a 4-year-old Palestinian girl was injured in the densely populated Gaza Strip.

Armed Islamist movement Hamas, which has ruled Gaza since 2007, took responsibility for the attack on the bus, saying it was retaliating for Israel’s killing of three Palestinian militants in an air strike on Saturday.

An Israeli F-16 warplane bombed a major Hamas security compound, rocking Gaza City with a big explosion and wounding at least one person there.

“We hope this situation will be contained, but we will not shy away from taking all the necessary action, offensive and defensive, to protect our country and to protect our citizens,” Israeli Prime Minister Benjamin Netanyahu told a news conference during a visit to Prague.

The Israeli military said 45 rockets and mortars were launched into Israeli territory from Gaza in the space of three hours, the heaviest fire in two weeks. There were no immediate reports of further Israeli casualties as a result.

Palestinian President Mahmoud Abbas urged Western powers to intervene “to stop this aggression,” the official Wafa news agency reported from Ramallah in the West Bank.

Abbas also urged Palestinian militants not to give Israel an excuse to hit Gaza.

ROCKETS INTERCEPTED IN MID-FLIGHT

In its first known combat launching of a system built to defend against cross-border rocket and missile attack, an Israeli interceptor missile destroyed in mid-flight two rockets fired from Gaza in the direction of the city of Ashkelon.

“Our Iron Dome anti-missile system intercepted two projectiles successfully,” Netanyahu said.

An Israeli helicopter gunship flying over Gaza machine-gunned a target there for the first time since the January 2009 war and fired a missile at some targets in the center of the coastal territory.

Helicopter machinegun fire killed a 17-year-old collecting scrap from the enclave’s derelict airport, medics said, and three others were killed nearby.

Palestinian sources said militants in the enclave fired back at the helicopter with a heavy machinegun.

The Israeli bus had been taking pupils home from school when it was hit by what a police spokesman said was an anti-tank missile fired from long range. Forty pupils had just got off the bus at a previous stop, he said.

The United States condemned the rocket attacks and expressed concern over the weapon used to hit the bus.

“We’re particularly concerned about reports that indicate the use of an advanced anti-tank weapon in an attack against civilians,” State Department spokesman Mark Toner said.

He noted that all countries have obligations under U.N. Security Council resolutions to prevent illicit trafficking in arms and ammunition.

Israeli Defense Minister Ehud Barak said he had ordered the military to “swiftly take all the necessary steps and respond to the attack” and Israel held Hamas responsible for all events taking place in the enclave.

The missile attack followed a relative lull in cross-border fire between Gaza and Israel after a sudden rise in violence last month in which at least 16 Palestinians were killed.

Israel and Hamas had signaled readiness to return to a de facto ceasefire which has kept the border relatively quiet since the end of the December 2008-January 2009 Gaza war.

A Hamas spokesman on Thursday repeated that his movement wants “calm” to return to the tense standoff.