US President Barack Obama and British Prime Minister David Cameron agreed meanwhile to plan a "full spectrum" of action on Libya, including a possible no-fly zone, surveillance and a relief effort.
The series of consultations came amid calls for immediate international action to halt Gaddafi's crackdown on rebels but with no clear unified world response to the strongman leader's defiance emerging.
Gene Cretz, the US ambassador to Tripoli who was in Washington before the uprising erupted in Libya last month, and other US officials met in Cairo with Libyan opposition leaders, State Department spokesman Philip Crowley said.
He declined to identify Cretz's interlocutors but said Washington has been in contact with opposition members inside and outside the national council, which is headed by former Libyan justice minister Mustafa Abdel Jalil.
"We are engaging a wide range of leaders, and those who both understand and can potentially influence events in Libya," Crowley told reporters.
"Cretz over the past few days was in Rome and Cairo for multiple meetings, both with Italian government officials, Egyptian government officials, but also opposition figures within Libya."
The talks with the Libya-based opposition sought to "gain a greater understanding and perspective on what's happening," Crowley added without elaborating.
He said Washington "will continue to watch the council," which is based in the eastern Libyan city of Benghazi but which he doubts has been fully formed.
"Eventually ... within Libya a formal opposition will emerge. We're watching to see how that develops," he added.
Michele Dunne, a former Middle East specialist at the State Department and White House, suspected that Washington hesitates to discuss its contacts with opposition leaders to prevent Kadhafi from portraying them as US "lackeys."
Dunne told AFP the United States is contacting opposition leaders to learn more about what kind of political institutions and system they want to build and who would be involved.
Unlike in Egypt and Tunisia, she said, Libya has no well-developed institutions, nor any kind of civil society, political parties and an independent media -- all of which makes it hard to discern future leaders.
"It's been a great mystery," said Dunne, now an analyst with the Carnegie Endowment for International Peace. "Should the rebels succeed in overthrowing... Kadhafi, who would be in control?"
US Secretary of State Hillary Clinton had several telephone calls Monday with foreign counterparts, including Italian Foreign Minister Franco Frattini and French Foreign Minister Alain Juppe about the opposition.
"We're sharing information and perspectives to fully understand the nature of the opposition and the course of events within Libya," Crowley said.
Showing continued US caution about a no-fly zone, Clinton told Sky News that any decision to impose such a measure to protect civilians from Kadhafi's air power should be taken by the United Nations and "not the United States."
Rebels have called for both a no-fly zone and weapons supplies.
Crowley said it would be illegal for any country to supply weapons to the rebels under a February 26 UN Security Council arms embargo, but said the international community could seek a waiver or amendment in the future.
But US Senators John McCain and Joseph Lieberman argued that the embargo applies to the Libyan government, not the entire Libyan population.
"We believe this language should be construed narrowly in order to hold open the possibility of providing military aid to the opposition," the senators said in a joint statement.
The White House said that Obama and Cameron "agreed to press forward with planning, including at NATO, on the full spectrum of possible responses."
Possible measures included surveillance, humanitarian assistance, enforcement of the arms embargo on Kadhafi's forces, and a no-fly zone, the White House said in a statement.
Britain and France have been drawing up a draft UN Security Council resolution on a no-fly zone which could be presented as early as this week.
But White House spokesman Jay Carney on Tuesday told reporters on Air Force One that though Washington was actively considering a no-fly zone, such a step involved "complexities."
Cameron and Obama also agreed that there must be an "immediate end to brutality and violence" in Libya and on the need for Kadhafi to leave power as "quickly as possible."