CAIRO - US Defence Secretary Robert Gates said on Wednesday there was no "timeline" for when UN-backed military operations in Libya would end, and that the outcome of the conflict remained unclear.

Speaking during a visit to Cairo, Gates said the UN Security Council resolution that authorised a no-fly zone was "not time-limited" and that it was unrealistic to expect military action to be over in a matter of weeks.

"So I think that there is no current timeline in terms of when it might end," Gates told reporters.

The military intervention was designed to prevent Libyan leader Moamer Gathafi from using aircraft to attack civilians and supply his forces, and coalition forces would monitor the effect of the strikes, he said.

"I think we will be assessing this as we go along, in terms of when his capabilities to do those things to his people have been eliminated.

"But I think no one was under any illusions that this would be an operation that would last one week, or two weeks, or three weeks."

The Pentagon chief also said it was difficult to gauge the strength of opposition forces because they had grown out of popular uprisings.

"I think it's been very hard for us to assess that frankly," he said when asked about the effectiveness of the rebels.

In the unrest that had erupted before international military action, Gates said "it wasn't as though you had an alternative army moving back and forth across Libya."

Some who had initially joined the uprisings, including troops from military bases, appeared to have withdrawn in the face of the regime's crackdown, he said.

But the Western air strikes might bring them back into the fray, said Gates.

"A lot of people who were in opposition, who played a role in the early days, have hunkered down.

"And it may be that changed circumstances, where he (Gathafi) can't use his aircraft and where he's more challenged in using his armour, they return to the fight. But we just don't know that."

Gates said the outcome of the conflict remained unclear, and that it was possible that more figures in Gathafi's regime could turn against him or even members of his own family.

"I think there are any number of possible outcomes here and no one is in a position to predict them," he said.

Possible scenarios included "further defections within his own ruling circle" or "divisions within his family."

His comments came on the fifth day of UN-backed military strikes against the Libyan regime as Gathafi vowed his country was "ready for battle."

Early Wednesday, CNN reported coalition air strikes were launched overnight near the city of Misrata, east of Tripoli.

Rebels said they had been under intense attack in their Misrata enclave, which has been besieged by Gathafi forces for weeks.

President Barack Obama faced fierce criticism over the US role in Libya.

Under pressure to bring US military strikes to a quick conclusion, Obama assured Univision television Tuesday that "the exit strategy will be executed this week" -- but made clear US forces would not really "exit" the conflict.

"We will be pulling back from our much more active efforts to shape the environment. We will still be in a support role, we'll still be providing jamming, and intelligence and other assets that are unique to us," he said.

"We put in place strong international sanctions. We've frozen his assets. We will continue to ply a whole range of pressure on him," he added.