James Clapper told the US Senate that Col Gaddafi's superior military force would prevail over the long term.

And Secretary of State Hillary Clinton said the US would not act in Libya without international authorisation.

Meanwhile, Libyan rebels are fleeing the port of Ras Lanuf after sustained attacks by Col Gaddafi's forces.

In Washington, Mr Clapper, who is the top intelligence adviser to US President Barack Obama, told the Senate he saw no evidence Col Gaddafi would step down from power and warned his military was stronger than had previously been described.

'Hunkering down'

"Gaddafi is in this for the long haul," Mr Clapper said.

"I don't think he has any intention, despite some of the press speculation to the contrary, of leaving. From all evidence that we have... he appears to be hunkering down for the duration."

In response to calls from some senior US Senate figures to establish a no-fly zone, Mr Clapper said Col Gaddafi's air defences were "quite substantial" and included Russian surface-to-air missile installations.

He said the Libyan military - which has an airforce of dozens of fighter jets, helicopters and transport craft - had attacked rebels but that the pilots "can't shoot straight" and had not inflicted many casualties.

Analysis contradicted

Also, Mr Clapper said one possible outcome could be the splitting of Libya into three autonomous states.

Later, Mr Obama's National Security Adviser Tom Donilon sought to temper Mr Clapper's remarks on Col Gaddafi's strength, suggesting his was a "static and one-dimensional analysis".

Mr Donilon told reporters the pressure of sanctions and the threat of the international community could overcome Col Gaddafi's military might.

Also on Thursday, the US revoked the diplomatic status of the Libyan envoys remaining in the embassy in Washington and suspended the embassy's operations.

Mrs Clinton said the US would meet representatives of the Libyan opposition, but emphasised the US would not undertake military action unilaterally.

"We're looking to see whether there is any willingness in the international community to provide any authorisation for further steps," she told a panel of the US House of Representatives.

"Absent international authorisation, the US acting alone would be stepping into a situation whose consequences are unforeseeable."