GAZA CITY, Aug 21 (AFP) - Israeli tanks and infantry early Wednesday moved into Khan Yunes in the south of the Gaza Strip, a Palestinian security source said, as violence erupted again after Israel left Bethlehem in a key test of confidence-building measures aimed at defusing the 23-month crisis.

One Palestinian was killed and four were injured when the troops dynamited a four-storey building, the source said.

The Israeli forces advanced with the tanks firing cannon and machine guns at houses, many in ruins, in the Palestinian town opposite the Gush Katif Jewish settlement block.

More than 20 armoured vehicles took part in the operation, with helicopters flying overhead.

The army said in a statement: "The army has undertaken a search operation in the suburbs of Khan Yunes after an outbreak of attacks in the sector." It recalled that an Israeli soldier was killed on Tuesday morning near the Neve Dekalim settlement by a Palestinian sniper hiding in Khan Yunes.

The Ezzedin al-Qassam Brigades, the armed branch of the Islamic Hamas movement, claimed the attack which was followed by an exchange of fire in which a young Palestinian was killed.

The Israeli army was ordered to "destroy abandoned houses that are used as shelter or as firing positions" by armed Palestinians, the statement said.

The latest death brought to 2,453 the number killed since the Palestinian uprising or intifada started in September 2000, including 1,808 Palestinians and 602 Israelis.

The violence occurred after the withdrawal Monday evening of Israeli troops from the West Bank town of Bethlehem, nearby villages and Dheishe refugee camp under an agreement reached Sunday with the Palestinian Authority.

The pullback took place under the so-called "Gaza, Bethlehem First" security plan concluded between Israeli Defence Minister Binyamin Ben Eliezer and Palestinian interior minister Abdelrazak al-Yahya.

The plan is the most significant security measure in almost a year, and foresees a progressive Israeli withdrawal from land it has re-occupied during the 23-month intifada, or uprising, if the reformed Palestinian security forces can ensure an end to attacks.

But just hours after the withdrawal from Bethlehem, the brother of a top Palestinian militant was shot and killed in a firefight in Ramallah Tuesday.

Mohammed Saadat, 22, brother of Ahmad Saadat, the jailed head of the Popular Front for the Liberation of Palestine (PFLP), was shot seven times as he tried to resist an Israeli special unit trying to capture him outside his Ramallah home.

The Damascus-based left-wing militant group later vowed revenge.

"This crime will not go unpunished," PFLP spokesman Maher Tahar told Nampa-AFP, adding that "Israel will pay dearly for its act."

Palestinian chief negotiator Saeb Erakat described the shooting as an "assassination" and called for the international community to send forces to protect Palestinian civilians.

The gun fight, which left an unknown number of Israeli soldiers injured according to witnesses, capped a day which saw two other Palestinians and an Israeli soldier killed, marring the cautious optimism that surrounded Israel's overnight pullback from Bethlehem, occupied for two months.

Israel fears that a single suicide bomber slipping through a roadblock to Jerusalem, just a few miles (kilometres) to the north would leave this initiative, like all it predecessors, in tatters.

Palestinian officials told Nampa-AFP that about 100 Palestinian police had arrived in Bethlehem in a convoy of about a dozen jeeps from nearby Jericho, the only main West Bank town not reoccupied by Israel.

If the plan works, it would be extended to other areas, starting with Gaza and nearby Hebron but hardline militant groups have vowed to thwart it amid an overall lull in the violence that has claimed more than 2,400 lives since September 2000.

A Palestinian militant from the Al-Aqsa Martys Brigades, a hardline offshoot of Yasser Arafat's Fatah movement, was killed and three other Palestinians were injured during clashes which erupted during an Israeli raid on a refugee camp in the northern West Bank city of Tulkarem.

An army spokesman said around 15 wanted Palestinians had been arrested during the operation, which lasted several hours.

In nearby Jenin, four Palestinian teenagers and an Irish woman peace activist were injured by gunfire as Israeli troops pushed into the town, Palestinian medics said.

Radical Palestinian groups and Israeli hardliners have rejected the withdrawal plan.

However, the US and neighbouring Jordan expressed hope the new security talks would help pave the way to a Middle East peace.

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