Palestinian officials warned Sunday night's airstrike — part of Israel's response to a suicide bombing last week that killed 21 people — would undermine efforts to rein in militants, a key requirement in a fading U.S.-backed peace plan.
"If the Israelis thought assassinations would destroy our determination to continue in our resistance, to continue defending ourselves, they are mistaken," Hamas spokesman Ismail Haniya said. "We will move ahead whatever the sacrifice."
The Hamas military wing, Izzedine al Qassam, issued a statement promising revenge.
"Our response will be painful and quick," it said in the statement faxed to The Associated Press in Gaza.
Meanwhile, Yasser Arafat appointed Jibril Rajoub to the vacant post of national security adviser, his latest move to outflank U.S.-backed Prime Minister Mahmoud Abbas in a struggle over control of security forces.
Abbas and his security chief, Mohammed Dahlan, have said they need to control all security forces to confront the militants. But Arafat has refused to hand over key units and has tried to have a loyalist, Gen. Nasser Yousef, appointed to the powerful post of interior minister. A meeting in Ramallah broke up late Sunday with no decision on the issue.
Saying the Palestinians have failed to crack down on militants, Israel has renewed its policy of hunting down and killing militant leaders.
In Gaza City, witnesses said Apache helicopters fired at least three missiles at a group of armed men near a crowded beach front Sunday night, killing four members of Hamas' military wing and wounding more than a dozen bystanders.
Some of the victims were decapitated by the assault, which took place just yards from the Gaza City office of Palestinian security chief Mohammed Dahlan.
"The first missile hit the car, four people fled the car and then three more missiles were fired at them. ... It was difficult to look at the scene," said Abdel Salam Abu Askar, a Palestinian journalist and eyewitness.
Israeli security officials said one of the dead, Ahmed Aishtawi, was a top operative who had planned and executed attacks in the Gaza Strip and West Bank.
A Hamas spokesman said Aishtawi was the leader of a unit that had pioneered the firing of homemade rockets into Israel and at Jewish settlements in Gaza.
Since November, when an Islamic Jihad ambush in the area killed 12 security guards and soldiers, settlers have repeatedly tried to set up an illegal outpost.
Under intense U.S. pressure to carry out commitments under the "road map" peace plan, Israel has dismantled several West Bank outposts. But Israel has failed to freeze construction at established settlements and to remove dozens of other illegal outposts.
Israeli police and soldiers early Monday removed about 50 settlers and a trailer from an illegal outpost near the West Bank city of Hebron and arrested five settlers there who refused to leave, Haaretz newspaper reported.
Settlers have tried several times to build an outpost along the so-called Worshippers Way, which connects the main Jewish settlement in Hebron and the nearby Jewish community of Kiryat Arba. Twelve soldiers and securities guards were killed there last November in an Islamic Jihad ambush.
Israel, meanwhile, has stepped up army sweeps through West Banks towns following a Hamas suicide bombing on a bus in Jerusalem that killed 21 people last Tuesday. Troops have been searching house-to-house for fugitives and weapons, sparking clashes.
Dozens of tanks and armored vehicles also have gathered along the Gaza border, apparently ready for an order to raid the coastal strip.
An Israeli helicopter rocket attack killed senior Hamas political leader Ismail Abu Shanab in Gaza City on Thursday.
Army chief Lt. Gen. Moshe Yaalon declared on Sunday that "every member of Hamas is a potential target for liquidation."
In the last three years of fighting, Israel has killed scores of wanted militants. Hamas and other militant groups called off a two-month cease-fire after Abu Shanab's killing.
Palestinian officials complained that the latest Israeli attack would undermine a planned Palestinian security clampdown that began Saturday with moves against arms smugglers.
"This aims to sabotage the efforts that began last night (Saturday)," said Saeb Erekat, a veteran Palestinian lawmaker.
Dahlan's forces, meanwhile, continued arresting weapons smugglers in the Gaza Strip, seizing arms and detaining at least 15 suspects in a sweep begun late Saturday. Security forces said they sealed off six tunnels used to smuggle weapons from Egypt to the Gaza Strip.
Despite Dahlan's activities, Palestinians fired about seven mortar shells overnight Sunday at Jewish settlements in the Gaza Strip, the army said. No one was injured.