Palestinian leaders, under pressure to clamp down on militants after a Hamas (search) bus bombing, were locked in a power struggle Sunday over command of their security forces, as Israel's army chief warned that every Hamas member is a "target for liquidation."

Meanwhile, Israeli helicopters fired missiles at a group of armed men in Gaza City (search) Sunday, killing four Hamas fighters, including a fugitive commander.

The crisis between Palestinian leader Yasser Arafat (search) and his rival, U.S.-backed Prime Minister Mahmoud Abbas (search), 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, Mohammed Dahlan, have said they need control over all men under arms to confront Hamas, Islamic Jihad and renegades in their own Fatah movement. Arafat stalled when asked to support such a crackdown after last week's bombing, which killed 21 people on a Jerusalem bus, including six children.

With the wrangling continuing on the Palestinian side, Israel intensified its hunt for militants, killing a Hamas leader, Ismail Abu Shanab, in a missile strike last week 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 the bus bombing.

"If at the end of the day, the Palestinian Authority doesn't move against them, we will have to do so," Yaalon said.

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

The United States made a rare appeal to Arafat last week to hand full control of Palestinian security forces to Abbas. Israel warned that time is running out before troops resume targeting militant leaders.

In a meeting of Fatah's Central Committee on Saturday, several members proposed appointing Gen. Nasser Yousef, a long-time Arafat loyalist, as overall commander of security forces.

The proposal was meant to make it easier for Arafat to give up control over the security services, participants said. It was also intended to sideline Dahlan, who is unpopular in Fatah's top circles and fell out with Arafat last year.

Arafat said he didn't mind appointing Yousef as Dahlan's boss, but balked at relinquishing control, participants said. The proposal was to be discussed again Sunday evening, but no resolution was expected.

As prime minister, Abbas also holds the role of interior minister, though Dahlan in effect has the job.

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

Appointing Yousef, with only some forces under his control, "is a wrong approach that changes nothing," Abbas told Israel TV's Channel Two.

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

Yousef, one of the oldest members of the Palestine Liberation Organization, was part of a guard force that spent the 1980s in exile with Arafat in Tunisia and Lebanon. In 1994, when the Palestinian Authority was established, Yousef was a senior police commander.

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.

The rocket strike came just hours after Dahlan's forces began arresting weapons smugglers in the Gaza Strip on Saturday evening, seizing weapons and detaining at least 15 suspects. Security forces said they also sealed off six tunnels used to smuggle weapons from Egypt to the Gaza Strip.

Arafat also appeared to flex his muscles Sunday, when a loyal commander toured Gaza. Palestinian officials said Abdul Razak Majadah ordered local security chiefs to maintain stability, including along Gaza's northern border.

Israel has gathered armored vehicles close to the border, where rockets and mortars have regularly been fired into Israel and at Jewish settlements in Gaza.

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

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.

Two rockets similar to those fired from the Gaza Strip were found in the explosives factory, the army and witnesses said. It is unusual for the army to find rockets in the West Bank, which is in closer range to central Israeli cities than the Gaza Strip.