On a day of new attacks and counterattacks by Israeli and Palestinian militant forces, diplomats said there was concern in the administration that without dramatic improvement of some kind, the peace initiative known as the road map could founder. 

A day after he criticized Israel for its attempt to kill a militant Palestinian leader, Mr. Bush today denounced a suicide bomb attack on a bus in Jerusalem that killed 16 people and wounded more than 100. 

"I strongly condemn the killings," he said, "and I urge and call upon all of the free world, nations which love peace, to not only condemn the killings, but to use every ounce of their power to prevent them from happening in the future." 

At a hearing of the House International Relations Committee, Representative Gary L. Ackerman, said that Mr. Bush's rebuke might lead his critics "to think of the word hypocrisy." 

"How can we take certain actions in response to terrorism, and then tell others that when they do the same exact thing that it is not helpful?" Mr. Ackerman, a New York Democrat, said during questioning of William J. Burns, the State Department's senior diplomat for Middle Eastern affairs. 

The influential pro-Israel lobbying group, the American-Israel Public Affairs Committee, known as Aipac, issued a rare criticism of Mr. Bush, if only obliquely. Israel, it said, "will and must take the responsibility to fight terrorist organizations" and "it should be the policy of the U.S. to support" such actions. 

The bombing today was an apparent retaliation for Israel's attempt to kill Dr. Abdel Aziz Rantisi, a top leader of Hamas, on Tuesday. It happened at about the same time as an Israeli missile attack in Gaza. 

Despite the violence, senior administration officials said that the peace initiative was still alive. 

Secretary of State Colin L. Powell and Kofi Annan, the United Nations secretary general met and urged calm. 

"There are those who do not wish to see the Palestinian people achieve a state living side by side in peace with Israel," Mr. Powell said. He called on Israel, the Palestinians and the Arab nations in the region to "remain steadfast, to continue moving down the path that was laid out at Aqaba last week by the leaders who were assembled." 

Mr. Powell was referring to a summit meeting in Jordan last week attended by Mr. Bush, Prime Minister Ariel Sharon of Israel and the Palestinian prime minister, Mahmoud Abbas. 

Mr. Annan called on the Israelis and Palestinians to "stay the course." 

But there was an unusual quality to the statements given the inflamed situation. If there was a new wrinkle to the day's developments, it was the criticism directed at Mr. Bush for his rebuke of the Israeli government on Tuesday. 

Reflecting dismay that a new round of violence might undermine the spirit achieved in Aqaba and Sharm el Sheik, Mr. Bush said the attack on the Hamas leader would not help Israel's security. His statement drew fire from those saying that Israel had carried out the attacks to defend itself, just as the United States has done. 

Representative Robert Wexler, a Florida Democrat, said Israel's use of military force to protect itself against "a ticking time bomb factory" was "100 percent justified." 

Representative Tom Lantos of California, the ranking Democrat on the International Relations Committee, defended Israel's right to protect itself, saying that the Palestinian Authority under Mr. Abbas was unable to do the job. If the Palestinians will not disarm terrorists, "then Israel clearly will do so," he said. 

"We would do so," he continued. "Any self-respecting society will do so. People in government have to defend their citizens." 

Appearing before the committee, Mr. Burns, the assistant secretary of state for Near Eastern affairs, acknowledged under sharp questioning from Democrats that Mr. Abbas would probably have to take forceful steps to disarm and dismantle terrorist groups if the peace initiative were to succeed. "I believe he is committed to doing the hard things that are going to be required to make that possible," Mr. Burns said. 

But a diplomat in touch with the administration said that the situation was so perilous that Mr. Abbas could be ousted from power if the cycle of violence did not abate. He said that there would be a meeting in Europe later this week of envoys focusing on the Middle East. 

The envoys — from the United States, the European Union, the United Nations and Russia — are to prepare for a higher-level meeting attended by Secretary Powell in Jordan on June 22. The group, known as the quartet, devised the step-by-step plan to end violence and establish a Palestinian state in three years. The plan was endorsed by the Palestinians and, in a qualified manner, by Israel last week. 

Despite the hopeful words from many sides, the fast-changing situation was putting new pressure on Mr. Bush to get more involved in saving the peace negotiations. 

On one hand, Israel sought today to dispute the American analysis that led the Bush administration to condemn the attack on the Hamas leader. Israeli officials said that far from being a mere political spokesman for Hamas, Dr. Rantisi was part of a faction within Hamas that advocated attacks on Israel as a means to destroy Mr. Abbas, who is also known as Abu Mazen. 

"There has been an ongoing debate within leadership circles in Hamas over the last few days about how to approach Abu Mazen," an Israeli official said. "One faction has said we have to fight against Abu Mazen and intensify terrorism. This faction lies outside Hamas itself — in Damascus, Jordan and elsewhere. Rantisi is one of their most vocal forces." 

But other diplomats said the Israelis had to know that the attack would provoke a new cycle of violence and make it impossible for Mr. Abbas to keep what little support he has among Palestinians.