GAZA CITY, Gaza Strip ? A missile fired from an Israeli helicopter killed at least 11 Palestinians today while they watched firefighters put out a fire in the Jabalya refugee camp in Gaza, witnesses and hospital officials said. More than 100 people were wounded.

The Israeli military operation in Gaza came hours after a Palestinian suicide bombing on a bus in the northern city of Haifa killed 15 Israelis and wounded dozens.

The army did not immediately comment on reports of the missile attack.

A gun battle had erupted in Jabalya during an overnight Israeli operation in the refugee camp, witnesses said, and continued as troops pulled out of the area and withdrew into an alley.

The troops fired tank shells and heavy machine guns. A helicopter hovering overhead fired missiles, hitting a group of people who were watching the firefighters at work.

“Until now, we have 11 killed and more than 100 wounded, among them 30 are in very critical condition, in a new massacre committed against the citizens of Jabalya,” said Dr. Moawia Hassanen, chief of emergency services at Shifa Hospital in Gaza City.

Earlier, two other Palestinians were killed during the army’s operation in Gaza. The army blew up two buildings, including one belonging to a Hamas activist.

A Reuters TV cameraman and photographer were among the wounded.

Israel has stepped up its operations in the Gaza Strip since mid-February, when Hamas claimed responsibility for blowing up a tank and killing its four-man crew. Days later a gunman killed a soldier.

Police and religious volunteers work at the scene of a bus bombing in the northern Israeli city of Haifa. A suicide bomber blew himself up aboard the crowded bus, killing at least 16 people and injuring dozens in the first suicide bombing in Israel in two months, officials said.

Since then, the army has raided several Gaza Strip towns and cities, including Hamas strongholds, in what it says is a hunt for militants and fugitives. More than 50 Palestinians, including militants and at least eight civilians, have been killed in more than a dozen raids.

Police and religious volunteers work at the scene of a bus bombing in the northern Israeli city of Haifa. A suicide bomber blew himself up aboard the crowded bus, killing at least 16 people and injuring dozens in the first suicide bombing in Israel in two months, officials said.

On Wednesday, a bomber blew himself up aboard a bus filled with students in this northern Israeli city Wednesday, killing at least 16 people and injuring 55. The blast ended a two-month lull in suicide bombings.

About 10 of the victims were high school students — among them 14-year-old Avigail Leitner, a U.S. citizen, authorities said. Two soldiers were also killed.

Police said the suicide bomber, Mahmoud Hamdan Kawasme, 20, of the West Bank city of Hebron, was carrying a letter praising the Sept. 11 attacks. No group claimed responsibility for the bus blast.

The bomb was laden with metal shrapnel for greater deadliness, according to Police Commissioner Shlomo Aharonishki. Initial reports said the blast was caused by 130 pounds of explosives.

A spokesman for the Islamic militant group Hamas, Abdel Aziz Rantisi, praised the bombing but did not claim responsibility. “We will not stop our resistance,” he said. “We are not going to give up in the face of the daily killing” of Palestinians.