After another bombing in Israel, the White House questioned Yasser Arafat's trustworthiness Wednesday and pledged to increase contacts with a new generation of Palestinian leaders who may be more willing to curb terrorism.

Israel's response to the suicide bombing — sending armored vehicles into Ramallah to surround Arafat's office compound — raised concerns at the White House that the violence could spiral.

Earlier in the day, White House spokesman Ari Fleischer made plain that "in the president's eyes, Yasser Arafat has never played a role of someone who could be trusted or who was effective."

A top Bush adviser said Fleischer's remarks reflect an increased U.S. emphasis on identifying potential alternatives to Arafat's leadership while at the same time leaving the door ajar to what Bush considers a remote chance that Arafat himself might reform the Palestinian Authority.

In a stern statement, the White House condemned the act of a suicide bomber who drove a car alongside an Israeli bus and set off an explosion. The Islamic Jihad claimed responsibility for the bombing, which killed at least 13 Israeli soldiers and several civilians.

"This attack underscores the fact that these terrorists are the worst enemies of not only the Israeli people, but also of the Palestinian people and their hopes for a better life," Fleischer said.

In contrast to the tough words for Arafat, administration officials were telling Israeli counterparts that Bush supported Prime Minister Ariel Sharon's right to defend his nation, although he should keep in mind the consequences of a harsh response.

One White House official said the United States was neither asked for nor did it grant a "green light" for the Israeli action in Ramallah, though it did not appear to be helpful in the push for peace.

Bush was notified of the Israeli action just before he joined a congressional barbecue on the White House lawn Wednesday evening, White House chief of staff Andrew Card said. His national security adviser, Condoleezza Rice, was looking into the situation.

Bush, dressed casually to greet his guests, waved off reporters' questions and did not mention the situation to gathered lawmakers.

State Department spokesman Richard Boucher said: "We've been following the situation closely. We've been in touch with both Israel and the Palestinians and getting updates" from U.S. diplomats in the area.

A senior administration official said the scale of Wednesday's attack underscored U.S. fears that Mideast terrorists are using increasingly sophisticated methods to kill. Israel said terrorists tried to use cyanide in a March 27 hotel bombing, but the deadly poison was not released because of a technical misstep.

On this score, Bush is concerned that Syria and Iran are helping Hezbollah modernize its weaponry as the group acts against Israel from southern Lebanon, the official said.

The sources spoke on condition of anonymity.

The latest attack complicates Bush's efforts to ease tensions with a fresh set of initiatives in time for a Mideast crisis conference in Turkey next month.

Administration officials are developing a number of options for Bush to consider, including proposing a timetable for peace talks and perhaps even some solutions to the thorniest political issues.

As part of the consultation process, Bush meets this weekend with Egyptian President Hosni Mubarak at the Camp David presidential retreat in suburban Maryland — and with Sharon at the White House on Monday.

Top Bush advisers said there are no plans to announce new initiatives immediately after those sessions.

Mubarak, who arrived Wednesday to begin talks with U.S. officials, wants Bush to demand that all parties agree to a deadline for Palestinian statehood. Bush has supported eventual statehood for Palestinians and will welcome Mubarak's advice, but he is unlikely to embrace the Egyptian's proposal before the Palestinian Authority moves to adopt democratic reforms and curb terrorism, said the senior administration official.

Increasingly, U.S. officials are openly questioning Arafat's relevance — specifically, his ability to reform the Palestinian Authority, curb terrorism and ease tensions with Israel.

"Chairman Arafat is the leader of the Palestinian Authority, but there are many other people who play constructive roles," Fleischer said. "Governments are made up of many individuals."

Secretary of State Colin Powell said Arafat faces pressure from his own people to reform.

"I think the Palestinian people expect Mr. Arafat to do more for them," he told National Public Radio. "They have now been in the intifada for a year and half and it hasn't brought them anything but grief."

Officially, Bush's position remains that the United States does not choose the Palestinian leader, thus Arafat is part of the peace process.

While some in the administration want the United States to sever ties with Arafat, there is a consensus inside the White House that Bush must keep in contact with the Palestinian leader to satisfy Arab leaders and to be ready in case he backs reforms. Meanwhile, the administration works on the assumption that Arafat won't adopt the necessary changes.

Assistant Secretary of State William Burns and CIA Director George J. Tenet are in the Middle East trying to bolster Israel's security against attacks and reaching out to Palestinian leaders other than Arafat, officials said.

Some reform-minded Palestinians are coming forward, while others are being courted, the Bush adviser said without naming names.

"What the president is interested in is results, from whatever corner they may come from," Fleischer said. "If that's Chairman Arafat, that's fine with the president; if it's others, that's fine with the president."

Sen. Joseph Biden, D-Del., chairman of the Senate Foreign Relations Committee, plans to meet next week with National Security Adviser Condoleezza Rice on the Mideast situation, which he said could hinder Bush's anti-terrorism campaign.

"The Middle East holds the key and/or is the blocking mechanism to a lot of other things," he said after meeting Bush on an unrelated topic.