Islamic militants killed an Israeli-American woman Friday just ahead of Yom Kippur (search), the first deadly shelling of a Jewish settlement in the Gaza Strip (search) in four years of Israeli-Palestinian fighting.

The attack, which came just hours before the start of the holiest day on the Jewish calendar, was likely to mobilize further opposition to Prime Minister Ariel Sharon's (search) plan to withdraw from the Gaza Strip next year.

The militant Hamas (search) group claimed responsibility for firing two mortars at the Neve Dekalim (search) settlement in southern Gaza.

A Hamas video showed three masked militants setting up and firing a mortar. After the shell was fired, heavy gunfire could be heard — apparently the Israeli return fire at the nearby Palestinian town of Khan Younis (search). The Israeli fire wounded two Palestinians, including a 4-year-old boy, witnesses said.

One of the Hamas mortars hit a house in Neve Dekalim, wounding two women who were taken to a hospital in southern Israel for treatment. One of the women, identified as Tiferet Tratner, died of her wounds, and the second was slightly hurt. Eran Sternberg, a settler spokesman in Gaza, said Tratner also held U.S. citizenship.

Palestinians have fired hundreds of crude mortars and rockets at Jewish settlements in Gaza and Israeli border towns since 2000, but Friday's attack marked the first time a resident of a Gaza settlement was killed. In June, two Israelis were killed in a rocket attack on the Israel town of Sderot.

Eli Moses, a resident of Neve Dekalim, said Sharon is to blame because he is going ahead with a plan to withdraw Israeli troops and settlers from Gaza. "We want to emphasize that the prime minister is directly responsible for the death," Moses told Israel Radio. "Whoever fired the mortar is of course responsible, but there's a person above him and unfortunately that's the prime minister."

The shelling came a day after Palestinians killed three Israeli soldiers in an attack on the nearby Morag settlement in southern Gaza. After a protracted gunbattle, the three attackers were killed by the army.

The Gaza clashes point to increasing tensions and violence ahead of Israel's planned withdrawal.

"The Palestinians are doing everything they can to scuttle an Israeli withdrawal from the Gaza Strip," said David Baker, an official in Sharon's office. "We will take any measures necessary to defend our citizens."

Neve Dekalim is one of 21 Gaza settlements, with a total of 8,200 residents, to be dismantled under Sharon's "unilateral disengagement" plan.

Sharon has said he believes Israel's presence in Gaza is untenable. He said evacuating the Gaza settlements and four isolated West Bank enclaves is a way of strengthening Israel's hold on parts of the West Bank, where most of its 236,000 settlers live.

Friday's violence came hours before Yom Kippur, or the Day of Atonement, when much of Israel shuts down for fasting and prayer. Israeli security forces have been on high alert since last week, the start of the Jewish Near Year, which ushers in a series of holidays that last until October.

Defense Minister Shaul Mofaz on Friday ordered troops to tighten a blanket closure of the West Bank and Gaza which has been in effect since the last week.

Since fighting erupted in 2000, Israel has restricted the entry of Palestinians to varying degrees, imposing closures during holidays or at other times of alert. In 2002, a Palestinian homicide bomber blew up a Passover gathering, killing 29 people.