The Bush administration is brokering a series of steps designed to unravel the regime in Syria but not oust the government of President Bashar Assad -- at least not yet, U.S. policymakers say.

Washington is intent on squeezing its most consistent nemesis in the Arab world to cooperate -- not only on Lebanon -- through the kind of pressure that eventually turned Moammar Gaddafi after Libyan agents were linked to the 1988 bombing of Pan Am Flight 103, according to U.S. officials.

The new United Nations report linking top Syrian officials, including Assad family members, to the killing of Lebanon's leading reformer eight months ago has sparked a "transformation" in how the world is willing to deal with Damascus, which Washington wants to cultivate, said a senior U.S. policymaker who spoke on the condition of anonymity because diplomacy is ongoing.

"Out of tragedy comes an extraordinary strategic opportunity," the official said. "This murder changed everything." Former Lebanese prime minister Rafiq Hariri was killed Feb. 14.

The long-term U.S. goal is to break the 35-year hold of the Assad family and allow Syrians to freely pick a new government. But in the short term, the administration is somewhat reluctantly opting to let the U.N. investigation and the subsequent judicial process, combined with punitive U.N. sanctions, erode Assad's power -- and see if he then changes Syrian practices in the region, U.S. officials said.

Damascus must end attempts to destabilize neighbors and undercut their aspirations, "whether that be the Lebanese people for independent sovereignty, whether it be the Palestinian people for an independent state that lives in peace with Israel, whether it be for the Iraqis who are trying to develop a peaceful and stable democracy but are having to fight a determined but unprincipled insurgency, or whether it be for Turkey," said State Department spokesman J. Adam Ereli.

In specific terms, that means such moves as closing the Damascus airport and border routes to extremists bound for Iraq, terminating ties with Palestinian rejectionist groups and the flow of arms to Lebanese militants, and developing good relations with the new Baghdad government, State Department officials said.

Washington is also talking with European allies about how to help Lebanon pursue the prosecution of those charged in the slaying, including holding trials with international help or at a venue outside Lebanon, U.S. officials said. Saad Hariri, Rafiq Hariri's son and the leader of the largest bloc in Lebanon's parliament, said yesterday that he wants his father's killers to be tried in an international court.

Although Syria has never been more isolated, U.S. officials caution that their ambitions are curbed by realities on the ground. After an intense hunt for alternatives, the Bush administration has concluded that there is no political party strong enough and sufficiently friendly to endorse as a replacement for Assad, U.S. officials said.

Unlike Iraq or Afghanistan, Syria has few democratic exile groups. The Muslim Brotherhood, an underground Islamist group, is the strongest internal opposition.

A more aggressive policy of "regime change" could backfire, U.S. officials said. An abrupt upheaval could invite a return to the kind of rampant instability and coups that typified Syria until Hafez Assad came to power in 1971.

"It's very difficult to identify someone viable within the power structure that the U.S. could work with other than Bashar. And if you look beyond the regime, the most likely alternative to the present political order would be heavily Islamist and anti-American," said Flynt Leverett, a former National Security Council staffer in the Bush administration and author of "Inheriting Syria: Bashar's Trial by Fire."

"And if this regime implodes, you would probably get chaos and violence along ethnic and sectarian lines . . . with spillover into Iraq and other parts of the region."

Yet Washington also believes that Assad -- an ophthalmologist who inherited power from his father in 2000 because his older brother, the designated heir, was killed in a car crash -- probably will not fully comply with the new terms laid out by the Security Council this week.

The Syrian leader vowed to punish anyone tied to the Hariri case. But he is unlikely to sacrifice his own brother, Maher Assad, or Gen. Asef Shawkat, his brother-in-law and the head of Syrian military intelligence, U.S. officials said.

Both were cited in the U.N. report by investigator Detlev Mehlis for allegedly planning the Hariri bombing. For his political salvation, Gaddafi turned over two agents tried for the Pan Am bombing. But the younger Assad has yet to demonstrate the confident pragmatism of his father, who sent troops to join the U.S.-led coalition to liberate Kuwait in 1991 and participated in the U.S.-orchestrated 1993 Madrid peace talks.

"For the past few years, there's been a deliberate effort to change Syria's behavior" -- with limited results, said Edward P. Djerejian, former U.S. ambassador to Syria. Then-Secretary of State Colin L. Powell was among several officials during the first Bush term who went to Damascus to seek common ground on terrorism, Lebanon, Iraq and the Arab-Israeli conflict.

Djerejian told Assad in January that Washington believed the time had arrived for Syria to finally act against Iraqis tied to Saddam Hussein in Syria. Assad took "half-measures" -- after the election -- reflecting his failure to understand how "to engage constructively and actively," Djerejian said.

And that may force the Bush administration to make a tougher decision, U.S. officials acknowledged. "The big question is: Is there anything to indicate that Assad would show any deviation from past behavior," said the senior U.S. official. "We're certainly not trying to save the regime."

And, unlike in Libya, Washington is unwilling to let the process of change take a decade, U.S. officials said. "Syria's situation is much more urgent," the senior official said.