Israeli troops pushed into the northern Gaza Strip on Thursday in a major campaign to stop Palestinian rocket attacks against Israeli targets. In brief clashes and sustained fighting, at least 19 Palestinians and three Israelis were killed, and more than 120 Palestinians and at least seven Israeli soldiers were wounded, according to Israeli military officials, Palestinian hospital officials and witnesses.

The fiercest battles occurred just north of Gaza City, in and around the Jabalya refugee camp, the largest refugee camp in the Gaza Strip and a stronghold of Palestinian guerrilla groups. Near Jabalya's marketplace and an adjacent school, a shell fired by an Israeli tank killed at least seven Palestinians and wounded 20, the highest casualty count of any incident Thursday. Another Israeli tank shell killed four Palestinians in the camp's eastern sector.

An Israeli military spokeswoman said the tank targeted a group of Palestinians who had fired an explosive device at Israeli troops, injuring three soldiers. Palestinian witnesses said the group consisted mostly of stone-throwers and bystanders.

In two days of fighting, at least 27 Palestinians and five Israelis have been killed.

Israel's incursion into Gaza follows a recently intensified campaign of missile and mortar attacks by Palestinian groups against Jewish settlements in Gaza and Israeli communities just outside the strip. Palestinians have said the stepped-up attacks are intended to show that, as Prime Minister Ariel Sharon pursues a plan to withdraw Israeli troops and settlers from Gaza, Israel is retreating from the area under fire.

Five Israelis have been killed in Palestinian rocket and mortar attacks in the last three months. On Wednesday, at the beginning of the Jewish holiday of Sukkot, two children, ages 2 and 4, were killed when a rocket slammed into an alley near their home in Sderot, a small Israeli industrial town less than two miles from the northeast corner of the Gaza Strip.

A senior Israeli official, speaking after Sharon met with his security cabinet late Thursday night, compared the situation in Gaza with the situation that existed in the West Bank following the March 2002 suicide bombing that killed 30 Israelis at a Passover seder at the Park Hotel in Netanya. That attack led to an Israeli military offensive in the West Bank called Defensive Shield.

"The situation cannot be tolerated anymore and something must be done to stop it, even if a high price has to be paid," said the official, who spoke on condition of anonymity. "We intend to begin a large-scale operation in Gaza" that will continue "until we achieve the desired results," he said.

The official said the operation would make it "hard, if not impossible, for Palestinians to launch rockets at Israeli towns and villages, and particularly at the city of Sderot."

He also said, "I hope this can be achieved in a short time, a few days," by clearing areas of cover and pushing Palestinian guerrillas away from the border. The longest range recorded for one of the Palestinians' homemade Qassam rockets, according to Israeli military officials, is just over six miles.

The official laid blame on the Islamic Resistance Movement, or Hamas, saying the organization wanted to create circumstances that would prevent Israel from withdrawing from Gaza "to give justification for their struggle, and if this situation continues, we might not be able to implement the plan. We will not be able to withdraw thousands of Israelis under fire."

Hamas, in a statement posted on its Web site, said: "The enemy understands only force and only the language of force. . . . Its wounds become deeper the more it roams in Gaza."

Israeli forces entered northern Gaza on Tuesday night. They occupied positions on major roads and in the fields outside several towns frequently used as cover for firing rockets over the heavily fortified border at Israeli communities.

Palestinian militants and civilians took to the streets to combat the Israeli incursion, witnesses said, sparking intense firefights. Despite the heavy presence of Israeli troops, several Palestinians successfully fired their projectiles, including the rocket that killed the two children in Sderot.

Thursday's fighting commenced at about 3:30 a.m., when two Palestinians from Hamas threw grenades and opened fire at soldiers manning an observation post overlooking Qassam launch sites in northern Gaza. The attackers killed one soldier, according to an army spokeswoman. Both attackers were killed in a subsequent exchange of fire, she said.

Later in the morning, two Palestinian gunmen shot and killed an Israeli woman who was jogging in a protected area between the Jewish settlements of Elei Sinai and Dugit in the northwest corner of the Gaza Strip, the army spokeswoman said. When Israeli troops came to the woman's aid, the spokeswoman said, an army paramedic was also shot and killed.

Israeli troops returned fire, killing one of the gunmen. The other led troops on a chase that lasted for several hours, ending when they shot him and he exploded, indicating that he was carrying explosive devices, the army spokeswoman said.

Four other Palestinians were killed in fighting Thursday morning and afternoon, but details were murky.

Special correspondent Islam Abdulkarim in the Gaza Strip contributed to this report.

Palestinians carry a wounded youth in the Jabalya refugee camp adjacent to Gaza City after Israeli tanks fired machine guns and launched shells into the area. An Israeli woman is comforted after learning that Palestinian gunmen killed a close friend of hers near the Elei Sinai settlement in northern Gaza Strip.