|Gaddafi's forces have retaken some of the key towns that had fallen to pro-democracy activists [EPA]
Ban Ki-moon, the UN secretary-general, has called on both sides in the conflict in Libya to accept an "immediate ceasefire", even as deliberations began at the United Nations Security Council over a draft resolution regarding the crisis.
Ban warned that "a campaign [by pro-Gaddafi forces] to bombard such an urban centre [as opposition stronghold Benghazi] would massively place civilian lives at risk".
He said he was gravely concerned about the increasing military escalation by government forces, which include indications of an assault on Benghazi.
"Those responsible for the continuous use of military forces against civilians will be held accountable," he said.
No-fly zone debate
The UN Security Council resumed discussions on Wednesday on a resolution seeking to impose a no-fly zone over Libya in a bid to stop Gaddafi's fighter jets from targeting civilians.
Al Jazeera's Scott Heidler, reporting from New York, said it would take the 15-member council sometime to conclude debate on the resolution.
"There are certain paragraphs - certain points within this draft resolution - that are going to be more sticky that others, if you will," he said.
"And that's specifically when it comes to the no-fly zone; how it is going to be implemented. There's a term 'by all means necessary'. That's quite ambiguous and also it can have a lot of military implications. That's most likely going to be one of those paragraphs that's going to take sometime to get through."
Saif: 48 hours from victory
Meanwhile, Saif al-Islam, Libyan leader Muammar Gaddafi's son, has said forces loyal to his father are nearing the rebel stronghold of Benghazi in the country's east and predicted an uprising against the regime would be crushed within the next two days.
Saif al-Islam told France-based TV channel Euronews on Wednesday: "The military operations are finished. In 48 hours everything will be over. Our forces are close to Benghazi. Whatever decision is taken, it will be too late."
He was responding to questions about talks among world powers to impose a no-fly zone over Libya, a move endorsed by the 22-member Arab League last week in Cairo and backed by the United States.
The interview was given as Gaddafi's forces launched a major attack on the rebel-held city of Misurata, killing at least five people, medical sources said.
Fighting was also under way in the eastern town of Ajdabiyah on Wednesday as rebels fought back against Gaddafi troops in an effort to halt their push towards Benghazi, the opposition's stronghold.
Rebels vowed to continue fighting even as government forces continued the counter-offensive which has seen them win control of a string of towns previously in opposition hands to the east of Tripoli.
"We've got some surprises in store. We're going to fight on and we're going to win," Mustafa Gheriani, a rebel officer in Ajdabiya, told the Reuters news agency by telephone.
State television claimed on Tuesday that forces loyal to Gaddafi had captured and "purged" the town.
Al Jazeera's Anita McNaught, reporting from Tripoli, said Gaddafi and his son have been increasingly confident of reversing the rebels' gains.
"There has always been confidence coming out of Colonel Gaddafi's mouth and out of the mouth of his son Saif al-Islam. Back - maybe more than three weeks ago - there was a little uncertainty because they were taken by surprise, I think, by the degree and speed of the uprising against them," she said.
"But as they had a chance to get a measure of what the rebels were able to put up in terms of a fight ... they have grown increasingly certain of how they could proceed to regain the territory they lost and how they could move against them [rebels]."
Al Jazeera's Tony Birtley, reporting from Benghazi, said the rebels would put up stiff resistance.
"I don't think there's too much nervousness, and I don't think people believe the threat at the moment appears to be real and looming," he said, adding that Benghazi would be a "whole different kind of target".
"This is a a huge city [with] a populaton of over 800,000 and the people here basically have been flying the flag of the rebellion from the very beginning.
"They know they have a lot to lose, so they're going to fight this every inch of the way, and street fighting would be potentially costly for Gaddafi's forces. We don't think he is that strong in terms of numbers ... There's a long way to go before Benghazi falls."
Rebels have long called for the imposition of a no-fly zone to protect them from aerial bombardment by Gaddafi's fighter jets. The measure is backed by the Arab League, France and Great Britain but other UN security council members have voiced doubts about the proposal.
Hillary Clinton, the US secretary of state, said on Wednesday she hoped the council would vote on a new package of measures against Libya as early as Thursday.
"We are moving as rapidly as we can in New York to see whether we can get additional authorisation for the international community to look at a broad range of actions, not just a no-fly zone but other actions as well," Clinton said on a visit to Cairo.
"We won't know until there is an actual vote. We're hoping that will be no later than tomorrow [Thursday]," she added.
"And then we'll see what that message means to Gaddafi and his regime, and what it means in terms of support and encouragement to the opposition."
Britain's UN ambassador, Mark Lyall Grant, said council members would discuss the proposed resolution "paragraph by paragraph" because members had "a number of questions about the text".
Colombia's ambassador, Nestor Osorio, said his country "very much supported" efforts to halt violence against Libyan civilians, but still had questions about the details of a possible no-fly zone.
Other ambassadors said issues to be clarified included whether the ban would apply to all flights countrywide, and what countries would contribute planes and other assets to enforce it.
Pro-Gaddafi troops have been battling rebels in several cities across the Libyan coastline, using machine guns and by airpower to halt the rebels' advance on key cities in the country's west.
The rebellion, which began last month in the relatively poor city of Benghazi, followed an Egypt-style uprising against Gaddafi, in power since 1969, and has seen hundreds of civilians killed, some by air force planes.
Source: Al Jazeera and agencies