Muammer Gaddafi's superior military forces meant his "regime will prevail" in the longer term, the US director of national intelligence, James Clapper, said in comments that undermined a robust defence by Washington of its Libya policy.
Mr Clapper said in testimony to Congress on Thursday that Colonel Gaddafi was relying on two of his brigades – which appeared to be "very, very loyal", "disciplined" and "robustly equipped with Russian equipment, artillery, tanks".
Mr Clapper, who oversees America's 16 intelligence services, said the rebels faced great difficulties as Col Gaddafi "intentionally designed the military so that those select units loyal to him are the most luxuriously equipped and the best trained".
He added: "We believe that Gaddafi is in this for the long haul. He appears to be hunkering down for the duration."
Tom Donilon, head of the National Security Council, said later that Mr Clapper was describing a "static situation" which did not take into account the "dynamic" change taking place throughout the Middle East.
Jay Carney, White House spokesman, separately rejected calls for Mr Clapper's resignation from a prominent senator, as financial sanctions, an arms embargo and the threat of military action "enhance the pressure on [Gaddafi] to force him to leave".
In his briefing on US policy, Mr Donilon said that military action remained on the table, including a no-fly zone, but that plans to deliver aid to rebel areas in coming days were purely humanitarian: "They can in no way shape or form be considered as military intervention," he said.
Mr Donilon emphasised that any action against Libya depended on support not just from the west, but also from other countries in the Middle-East. He added: "Military steps are not the only method by which we can pressure Gaddafi."
In relation to opposition groups, Mr Donilon said the US was "directly engaged" in an effort to understand their organisation, structure and leadership. The US has withdrawn recognition from the Libyan embassy in Washington but has not said whether it intends to, like France has, recognise the Libyan opposition.
Hillary Clinton, US secretary of state, will meet members of Libya's opposition when she visits Egypt and Tunisia next week.
"I will be meeting with some of those figures when I travel to discuss what more the United States and others can do," she said in testimony to Congress.
Ms Clinton had been more hawkish than the White House in canvassing possible US response to the violence in Libya, especially in expressing support for a no-fly zone. However, the White House has shown no appetite for rushing into a military response.
Mr Donilon said the US policy towards Libya was part of an overall response to ensuring the upheaval in many Middle-Eastern countries worked for the benefit of their populations.
The US was "very focused", he said, on the economy in countries such as Egypt, as that was the key to strengthening and entrenching representative democracy.