Forces loyal to Muammar Gaddafi, the Libyan leader, have shelled rebel positions in the western mountains, after opposition fighters edged closer to the capital on three fronts.
A rebel spokesman in the town of Nalut said there were no casualties from the shelling on Wednesday.
"Gaddafi's forces bombarded Nalut ... Over 20 Grad rockets landed in the town. They bombarded from their positions ... around 20km east of Nalut," he said, adding that they had also shelled the Wazin-Dehiba Tunisia border crossing.
The rebels have made advances in key areas in recent days but still have to overcome heavy odds if they are to successfully march on Gaddafi's well-defended territory.
The rebels said NATO leaflets warning of helicopter strikes had prevented them from pushing towards Zlitan, and had prompted some rebels to retreat from their newly-captured positions about 10km outside the city, towards their base in Misurata, east of Tripoli.
A NATO official said the alliance dropped the leaflets west of Misurata, and closer to Zlitan. A Reuters news agency correspondent in Misurata said there were no further advances towards Zlitan on Wednesday.
NATO war planes bombed Tripoli, the capital, on Tuesday night, causing loud explosions that filled the sky with plumes of smoke, but there were no further reports of bombing on Wednesday.
On Tuesday, the rebels tried to advance in the east, setting their sights on the oil town of Brega to extend their control over the region, epicentre of the four-month rebellion against Gaddafi's four-decade rule.
In a sign that Gaddafi's forces may be getting stretched, the rebels seized the town of Kikla, 150km southwest of Tripoli.
'No ground forces needed'
The push by the rebels came as the NATO commander heading the Libya operations said that the alliance's mission "can be completed without the use of ground troops".
Lieutenant-General Charles Bouchard also said that the military situation in western Libya was developing "very positively".
"I do believe we can complete the mission without bringing in ground troops," the Canadian general told reporters off Libyan shores on the Garibaldi, the flagship for NATO's maritime embargo on Libya.
"My view is that this operation has clear support from the North Atlantic council and we are receiving adequate assets to complete the mission and carry out our mandate," he said, adding: "We don't have a mandate for regime change."
Anders Fogh Rasmussen, the NATO secretary-general, was set to meet with David Cameron, the British prime minister, and William Hague, the foreign secretary, later on Wednesday for talks on the three-month military operation.
Senior military officials from Britain and France, key players in the NATO campaign, had earlier expressed concerns about how to maintain the NATO operation, just extended for three months from June 27.
Robert Gates, the US defence secretary, last week issued a stinging rebuke to NATO allies, warning their reliance on US military might is putting the Libya mission and the alliance's very future at risk.