The United States is cautiously optimistic about the immediate prospects for a post-Gaddafi Libya, while acknowledging enormous challenges lie ahead as the war-torn country faces the task of filling a political vacuum after decades of rule by one man, analysts said on Monday.

On Sunday night, Libyan rebels entered the capital Tripoli, and have as of Monday afternoon taken control of about 90 percent of the city, according to the U.S. State Department. The development signaled a near end to Libyan leader Muammar Gaddafi's 40-year despotic rule.

Despite rebels' jubilance and what international media reported as the celebration of supporters who chanted "It's over, frizz-head," - a nickname meant to mock the deposed leader - a number of challenges remain.

"With the lid off after 40 years of tyranny, there is the potential for quite a bit of lawlessness unless opposition leaders emerge - especially in large portions of the newly fallen west," said Wayne White, a scholar at the Middle East Institute who for decades served in various U.S. government positions in the Arab world.

Another potential hurdle is relations between the National Transitional Council (TNC) and factions elsewhere in the country. More immediately, Gaddafi's whereabouts need to be resolved, White said.

"Will he try to flee to another part of Libya, such as his hometown bastion of Sirte to rally support? Could angry supporters attempt to take parting revenge against the opposition by attacking and torching oil wells in the deep desert of the east and east-central part of the country?" White said.

Christopher A. Preble, vice president for defense and foreign policy studies at the Cato Institute, wrote on the organization's Web site that the most important issue is figuring out who will replace Gaddafi.

"We might know more about the rebel movement than we did in March, to the extent that the TNC represents them. But does it?" he asked, adding that it also remains unknown whether the organization boasts sufficient support to govern the entire country.

Speaking from Martha's Vineyard on Monday, U.S. President Barack Obama said the situation is still very "fluid," adding that there remains a degree of uncertainty.

"But this much is clear: The Gaddafi regime is coming to an end, and the future of Libya is in the hands of its people," he said.

US State Department spokeswoman Victoria Nuland told reporters in Washington on Monday that the United States remained "cautiously optimistic" in the immediate aftermath of Gaddafi' s fall.

Nuland said Libya's National Transitional Council has put much thought into the issue, although she did not say whether there were any specific plans on the table for a post-Gaddafi government or society.

"We've also been cautiously optimistic by the situation that we've seen in the liberated parts of Tripoli so far, but this is certainly something that we are watching...because we don't need any more civilian life lost," she said.

The United States and the international community are working with the TNC as the group works through its own plans for the transition, which include trying to ensure public safety and essential services. "This must be and will be a Libyan-led transition," Nuland said.

What lessons has the United States learned from Gaddafi's ouster?

While Gaddafi's ouster is a good thing for Libya, the lessons that are likely to be drawn from it, especially those of the U.S. role, might not be good for the United States, Preble argued.

The Libya story will be fit into a familiar narrative, in which the United States is portrayed as uniquely suited to be the world's government, with the U.S. military as a global constabulary, responding to threats large and small, distant and proximate, he contended.

"Most Americans disagree. Such an approach to the world has taxed our military, and overburdened U.S. taxpayers, with no obvious benefit to U.S. national security," Preble argued.