Jerusalem ? U.S. envoy Anthony Zinni launched his third Mideast truce mission on Thursday, prompting Israel to withdraw forces from a key West Bank town. The visit came during a major Israeli offensive and the bloodiest period in 18 months of Israeli-Palestinian fighting.

After Zinni met Israeli Prime Minister Ariel Sharon in Jerusalem, the Israeli troops began a steady pullout from Ramallah. Witnesses said the tanks were leaving from all directions.

Israel Radio said it would take several hours because of the large number of soldiers and tanks involved in the Ramallah incursion, part of Israel’s largest military operation since the 1982 invasion of Lebanon.

The withdrawal eased some of the tension at the beginning of Zinni’s mission, but it did not guarantee success. Casualties mounted and each side warned that it would continue fighting if the other did not end the violence.

Israeli forces killed five armed Palestinians in gunbattles and two militiamen in a helicopter attack in Gaza, while Palestinian militants set off a bomb under an Israeli tank, killing three soldiers.

Zinni met with Prime Minister Ariel Sharon Thursday night.

“My main effort now is to achieve a cease-fire,” Sharon told reporters before the meeting at his Jerusalem residence, across the street from a cafe where a suicide bomber killed 11 people last Saturday.

Palestinian officials said Zinni was to meet with Arafat on Saturday.

As Zinni arrived, Israeli Defense Minister Binyamin Ben-Eliezer ordered a gradual pullback of troops from Ramallah, the Palestinian commercial center in the West Bank that has been occupied by dozens of Israeli tanks since Tuesday.

But the Bush administration said that was not enough.

“We do expect a complete withdrawal from Palestinian-controlled areas, including Ramallah,” State Department spokesman Richard Boucher said. He also called on Arafat “to do more to stop the groups that carry out violence.”

Britain’s Foreign Office also contacted Israel’s government and urged it to pull out of Ramallah, saying Israel should take “immediate steps to de-escalate” its operations.

Later in the day, the pullback from Ramallah began.

The Israeli army chief of staff has said 20,000 soldiers have been deployed in the West Bank and Gaza Strip in response to a string of attacks on Israelis by Palestinians.

In all, 174 people have been killed on the Palestinian side and 62 people have been killed on the Israeli side since the beginning of March.

Early Friday, Israeli tanks remained in neighborhoods in Bethlehem. they had entered the city from all directions in the early morning hours Thursday. Some tanks were parked 300 yards from the Church of the Nativity, traditional site of the birth of Jesus.

In Bethlehem, Palestinian militiamen shot dead two Palestinians suspected of being informers for Israel. The gunmen then dragged one of the bodies through the streets on the back of a pickup truck and tried to dangle it from a building on Manger Square, overlooking the Church of the Nativity.

One of the slain men was sentenced to death early last year for allegedly helping Israel kill a Palestinian militia leader; the other had not been tried.

Also in Bethelehem, an Israeli tank shell slammed into a church, showering shrapnel on a statue of the Virgin Mary, slicing off the hands and nose, a nun at the church said. Israel said it was investigating.