Hours after Israel's army chief said Hamas militants were targets for "liquidation," Israeli helicopters fired missiles at a group of armed men sitting near the Gaza City (search) beach Sunday, killing four Hamas (search) fighters, including a fugitive commander.

"Israel has no choice but [to] act in those areas where the Palestinians are failing to do so," said Gideon Meir, a senior Israeli Foreign Ministry official.

The attack occurred just 200 yards from the office of Palestinian security chief Mohammed Dahlan (search).

Israel's army chief said only hours earlier that all members of the Islamic militant group are "potential targets for liquidation." Sunday's attack came three days after Israel killed Hamas leader Ismail Abu Shanab in a similar strike in retaliation for a Hamas bombing that killed 21 people, including five Americans, on a Jerusalem bus.

Palestinian officials said Sunday's attack would undermine a planned Palestinian security clampdown that began Saturday with moves against arms smugglers, casting fresh doubt on an already shaky U.S.-backed peace plan. That so-called "road map" envisions a Palestinian state by 2005.

"This aims to sabotage the efforts that began last night," said Saeb Erakat, a senior Palestinian lawmaker. "It's very obvious that the Israeli government is acting as if the Palestinian Authority (search) is something from the past."

One witness, Shadi Wassi, said he was about to enter his house "when suddenly a huge explosion shook the ground under my feet. When I looked back, I saw a big flame burning the trees, then another two huge explosions hit the area."

Other witnesses said the men were sitting near the beach for about half an hour when the missiles hit.

Bystanders carried the bloodied body of one man to an ambulance, as the helicopters fired flares. Onlookers holding cigarette lighters searched the ground to gather pieces of flesh from the sand.

Hamas identified the dead men as fighters Ahmed Aishtawi, Wahid Hamas, Ahmad Aub Helal and Mohammed Abu Lubda.

An Israeli military official said on condition of anonymity that Aishtawi, 24, was the main target, describing him as a senior operative who planned and committed attacks in Gaza and the West Bank.

A Hamas spokesman said Aishtawi led a unit that pioneered the firing of homemade missiles and specialized in hitting tanks.

Aishtawi's 21-year-old brother, Hussam, said, "I am sad because I lost my brother, but I am happy because he became a martyr. I will follow in his footsteps."

The military strike came as Palestinian leaders were locked in a power struggle over command of their security forces.

The crisis between Palestinian leader Yasser Arafat and his rival, U.S.-backed Prime Minister Mahmoud Abbas, was triggered by Arafat's refusal to relinquish control of security forces as demanded by the United States in a push to dismantle armed groups.

It appears unlikely Arafat will back down since his authority would be considerably weakened if he gives up command over security. He controls several key security branches, while Abbas controls the rest.

Abbas and his security chief, Dahlan, have said they need control over all men under arms to confront Hamas, the smaller Islamic Jihad militant group and renegades in their own Fatah movement. Arafat stalled when asked to support such a crackdown after last week's bus bombing, which killed six children.

Several members of Fatah's Central Committee have proposed appointing Gen. Nasser Yousef, a longtime Arafat loyalist, as overall commander of security forces.

Arafat said he didn't mind appointing Yousef as Dahlan's boss, but balked at relinquishing control, several committee members said.

Israel has accused Arafat of involvement in terrorism, and the United States has ignored him for months, seeking instead to work with Abbas, who was appointed in April under U.S. pressure.

Abbas on Sunday stood by Dahlan, and said he will not resign as security chief.

As the Palestinian wrangling continued, Israel intensified its hunt for militants, killing Shanab on Thursday and sending troops and tanks into West Bank towns.

"Every member of Hamas is a potential target for liquidation," Israeli army chief Lt. Gen. Moshe Yaalon said Sunday in the first public comment by a senior defense official on Israel's new policy, adopted after Tuesday's bus bombing.

In the past three years of fighting, Israel has killed scores of wanted militants in targeted attacks -- the Palestinians call them assassinations -- but rarely has gone after Hamas political leaders. Abu Shanab was the most senior Hamas leader killed in a missile strike.

Hamas remained defiant after Sunday's missile strike.

"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."

Also Sunday, a rocket fired from the Gaza Strip landed about four miles from the Israeli city of Ashkelon, about 10 yards from an unmanned lifeguard post. It was the deepest a Palestinian rocket has struck in Israel in recent memory, the army said.

Dahlan's forces, meanwhile, continued arresting weapons smugglers in the Gaza Strip, seizing weapons 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.

Israeli security officials dismissed the Palestinian raids as fiction and affirmed that Israel will continue acting against militants, a security source said Sunday.

In the West Bank city of Nablus, the Israeli army said it uncovered a bomb lab Sunday, blowing up the site where they found a 176-pound bomb and bomb-making materials.

The Associated Press contributed to this report.