Israeli soldiers check 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. (BAZ RATNER/REUTERS)

An antitank missile fired from the Gaza Strip hit a school bus in southern Israel on Thursday, critically wounding a teenage boy and prompting Israeli helicopter and artillery strikes that killed four Palestinians and wounded dozens more, military and medical officials said.

The bus driver was lightly wounded in the attack and a 16-year-old passenger, the last on the bus, was wounded in the head. A spokeswoman at Soroka Hospital in the southern city of Beersheba said the teenager was in critical condition late Thursday.

A military spokesman said initial findings indicated that the bus had been struck by an advanced Russian-made Kornet antitank missile that had been fired from about two miles away. Militants in Gaza have fired antitank rockets at Israeli tanks and armored vehicles, but Thursday’s attack was the first in memory in which such a missile struck a civilian vehicle.

The Izzedine al-Qassam Brigades, the armed wing of the militant group Hamas, which rules the Gaza Strip, asserted responsibility for the attack, calling it retaliation for an Israeli airstrike that killed three senior Hamas operatives last Friday.

Fawzi Barhoum, a Hamas spokesman, said the attack was an act of “self-defense to protect Palestinian civilians and citizens.”

Israeli artillery and helicopters struck back at targets across the Gaza Strip, killing a 50-year-old man in Shajaiya, east of Gaza City, and three other Palestinians in the southern town of Rafah, according to a spokesman for Gaza’s emergency services. He said 39 other people were wounded. The Qassam Brigades identified one of the dead in Rafah as a member of the group.

The Israeli army said that 45 mortar rounds and rockets landed in Israel during the exchanges that followed the bus attack, causing no casualties. In a first operational success, Israel’s Iron Dome missile defense system shot down a Grad rocket headed for the southern coastal city of Ashkelon, a few miles north of the Gaza Strip, the army said. The anti-missile battery had been deployed in the area this week in anticipation of further hostilities.

Prime Minister Binyamin Netanyahu, on a visit to Germany, said Israel would not tolerate cross-border attacks from Gaza. “We have no intention of absorbing attacks that no country would accept,” he said.