RAMALLAH, West Bank ? Israel tightened its siege on Yasser Arafat late Friday, using tanks to destroy a stairwell in his compound, digging a deep trench and running coils of barbed wire around his offices.

The scope of the Israeli action made it apparent that Arafat was caught in Israel’s tightest chokehold yet. U.S. officials urged Israel not to go too far in its reprisal for a Tel Aviv bus bombing that killed six people.

Smoke billows above Palestinian leader Yasser Arafat's compound, as an explosion set off by Israeli army ground troops destroys part of the besieged leader's headquarters in the West Bank town of Ramallah. The explosion was the third in the compound Friday, coming a day after a suicide bombing in Tel Aviv.

The compound resounded with gunfire sporadically throughout the day. The Israelis blew up three buildings in the morning and three loud explosions were heard hours later. Much of the compound was in ruins both from the day’s violence and an earlier siege.

Nabil Abu Rdeneh, an aide to Arafat, said Israeli troops fired several tank shells at the stairwell in the section where Arafat was, to prevent people from moving between the first and second floor.

The U.N. Security Council called a meeting on the violence for Monday morning at the Palestinians’ request.

One tank shell destroyed the stairs to the ground floor below Arafat’s quarters, and Israeli snipers took up positions in windows facing the rooms, Abu Rdeneh said. Two more shells were fired at another section of the building, he said.

“President Arafat and those with him are in danger,” Abu Rdeneh said, adding that he believed the building could collapse.

With the demolition of the walkway between the two sections of his office, Arafat and a few associates, along with about 20 wanted men, were isolated in one area and separated from most of his guards in the other section, Abu Rdeneh said.

Twenty-seven guards surrendered to Israeli troops, holding up their shirts to show they weren’t carrying weapons or explosives.

The White House and the European Union urged Israel to show restraint, suggesting that too harsh a reprisal for a Tel Aviv bus blast claimed by Arafat’s Islamic militant rivals would upset quiet efforts to reform the Palestinian Authority and secure a truce. Six people were killed in Thursday’s bus attack.

Israel said troops would only withdraw after the surrender of the 20 wanted men, who include West Bank intelligence chief Tawfik Tirawi. Arafat’s aides said he would not hand over anyone to the Israelis.

Enraged by the bus attack, Israeli Prime Minister Ariel Sharon reportedly raised the idea of expelling Arafat at an emergency Cabinet meeting Thursday. Defense minister, Binyamin Ben-Eliezer, said the plan for now was to isolate, not oust the Palestinian leader.

However, TV reports said the goal of the current assault was to make Arafat seek exile voluntarily, by confining him to a tiny area and making life in the compound unbearable. Ben-Eliezer, arguing that an outright expulsion is counterproductive and would only boost Arafat’s standing, proposed that plan to Sharon in the Cabinet meeting, TV’s Channel Two said.

Arafat has said he would never again leave the Palestinian lands.

Five Palestinians died and 25 others were wounded during Israeli military action Friday. The dead included an Arafat bodyguard shot by snipers in the Ramallah compound.