The attacks came as Israel said it was widening its campaign against Hamas by targeting the group's political as well as military leaders.
In the past four days, Israel has carried out seven missile strikes against Hamas targets.
Meanwhile, in the West Bank, one Israeli was killed and two were seriously wounded in Palestinian shooting attacks.
Friday's deaths meant 36 Palestinians and 24 Israelis have been killed since President Bush launched his Mideast "road map" to peace last week at a summit in Jordan.
Concerned over the escalating violence, U.N. Secretary-General Kofi Annan said in an interview published Friday that deploying armed peacekeepers — an idea opposed by Israel — may be the only way to keep the warring parties apart long enough to begin implementing the so-called "road map" to Mideast peace and Palestinian statehood by 2005.
The plan envisions an increased role for international monitors, but does not call for armed peacekeepers.
A first contingent of 10 to 15 U.S. monitors, including CIA and State Department officials, was to arrive in the region on Saturday. The group is headed by John Wolf, an assistant secretary of state.
Israel TV reported that Dov Weisglass, a senior aide to Israeli Prime Minister Ariel Sharon, is flying to Washington for talks with National Security Adviser Condoleezza Rice on the crisis.
Palestinian Prime Minister Mahmoud Abbas has said he won't use force against Hamas and other militants and needs time to persuade them to lay down arms. Egypt is mediating, and talks might be held next week in Cairo between militant leaders and Egyptian intelligence chief Omar Suleiman, Palestinian officials said.
In the past 32 months of fighting, Israel has killed more than 100 wanted Palestinians in targeted attacks, including many from the Hamas military wing. The group's top political leaders were largely left alone, possibly because of Israel's fear of a bloody backlash.
After Hamas' rejection of a cease-fire, however, the group's leaders were marked for death, said the Israeli security official. They are considered ``ticking bombs'' and therefore legitimate targets because they set policy and order attacks on Israelis, the official said.
``There is no immunity for anybody who either orders or executes terrorist activities,'' said Avi Pazner, a government spokesman.
Military correspondent Alex Fishman wrote in the Yediot Ahronot daily Friday that ``from now on, everyone is in the crosshairs all the time,'' including Yassin.
In a first such strike, Israel tried this week to kill Abdel Aziz Rantisi, a Hamas co-founder and spokesman. Rantisi escaped a missile strike with minor injuries, but in response, a Hamas bomber blew himself up on a Jerusalem bus a day later, killing 17 people.
Israel has targeted members of the Hamas military wing in six rocket attacks this week, the latest on Friday. Helicopter gunships on Friday fired three missiles at a car carrying Hamas operatives who, according to the military, had launched homemade rockets at Israel earlier in the day.
The missiles hit the car as it was driving in the Sabra neighborhood of Gaza City, killing a Hamas militant identified as Fuad Ledawi. At the time, the street was crowded with worshippers walking to a nearby mosque for evening prayers. Three of the 32 wounded were in critical condition.
Though the government insists such attacks are the only way to stamp out the militants, an Israeli opinion poll suggested the new policy is unpopular.
Sixty-seven percent of 501 respondents said Israel should halt targeted killings, at least for a while, to allow Abbas to strengthen his shaky position and act against the militants. The poll published in Yediot had a margin of error of 4.5 percentage points.
The road map requires Abbas to dismantle and disarm militias, but he has said he cannot and will not use force against them for fear of setting off a civil war.
A vacationing President Bush, on the golf course with his father in Maine, declined to answer reporters' questions Friday morning about whether the "road map" peace plan was in trouble, reports CBS News White House Correspondent Mark Knoller.
Secretary of State Colin Powell insisted the Bush administration's Middle East peace plan is not dead yet.
"We've got to punch our way through it and get on with the steps called for in the road map," he told reporters outside the State Department Friday. "We have a road map. Both sides know what their obligations are, and we're going to drive forward on the road map."
Powell will meet in Jordan next week with other members of the so-called "Quartet" of Mideast mediators — the European Union, United Nations and Russia.
Powell called on Abbas to work harder to rein in militants. "We want him to use that limited capability as effectively as he can," Powell said.