DAFNIYAH, Libya (Reuters) - Libyan rebels and pro-Gaddafi forces exchanged heavy artillery fire near the western city of Zlitan on Friday as the rebels tried to push deeper into government-held territory east of the capital.
A Reuters team in Dafniya, the outskirts of the rebels' western bastion of Misrata, described rebels firing artillery and rocket launchers with a range of about 20 kilometers (miles). Rebels said they were aiming for tanks and munitions in Naimah near Zlitan.
"We had a strategy to finish everything today but some of the fighters think it's a game," a rebel unit commander called Mohammed Ali told Reuters. "They shot when they weren't supposed to shoot and they have ruined it," he said after rebels took cover at the main Dafniya front from heavy mortar barrage.
Warplanes could also be heard in the skies above, although it was unclear if there had been strikes.
Zlitan, just 160 km (100 miles) from Tripoli, is the next major town on the Mediterranean coast road to the capital. Capturing it would be a major victory.
The exchanges were the heaviest in the area since last week when 31 rebels were killed.
At the field hospital in Dafniyah ambulances arrived with at least 5 seriously wounded rebel fighters, most with shrapnel wounds.
The rebels have said they would not attack Zlitan because of tribal sensitivities, but have been recruiting fighters from the town and waiting for the inhabitants to rise against Muammar Gaddafi.
NATO planes also resumed bombardments of Tripoli on Friday with six loud explosions ringing out in the south of the city.
The rare daytime strikes, which hit the capital over the course of about half an hour before noon, sent columns of thick black smoke into the sky.
The rebellion rose up four months ago to the day in the eastern city of Benghazi and NATO intervention in Libya has now been going on for nearly 13 weeks -- longer than many of its backers anticipated -- and the strains are beginning to show within the alliance.
French Military spokesman Thierry Burkhard suggested the rebels were honing in on Gaddafi's stronghold of Tripoli.
"The opposition forces seem to have taken the ascendancy on Gaddafi's troops, which shows just how much attrition they are enduring," he told reporters on Thursday.
The rebel advance, he said, was "essentially in the West and in a belt they are now developing around the Tripoli region."
Rebel advances toward Tripoli have been slow, while weeks of NATO strikes pounding Gaddafi's compound and other targets have failed to end his 41-year-old rule.
Rebel forces are fighting Gaddafi's troops on two other fronts: in the east of the country around the oil town of Brega and in the Western Mountains southwest of Tripoli.
Juma Ibrahim, a rebel spokesman in Western Mountains town of Zintan, said Gaddafi loyalists were massing in Gharyan, about 120 km (90 miles) south-west of Tripoli.
"Battles continued today at the Tekot area (near Nalut) between pro-Gaddafi forces that used Grad missiles and tanks to shell the positions of rebels," he said, adding NATO strikes in the last 48 hours had been "very helpful".
They have made slow but important gains in the past few weeks in the mountains and near Misrata, bringing the front closer to Tripoli from the east and southwest.
Rebels said late Thursday an attack on their positions near the town of Ajdabiyahwounded at least 16 fighters in what may have been a friendly fire incident caused by a NATO air strike.
NATO said it was investigating the incident.
One of Gaddaf's sons Saif al-Islam told an Italian newspaper on Thursday the elections could be held within three months and his father would step aside if he lost, but that proposal was swiftly rejected by the rebel leadership and the United States.
Libyan PM Al-Baghdadi Ali Al-Mahmoudi appeared to throw Saif al-Islam's potential concession into question, saying the leader of the revolution was not concerned by "any referendum".
A visiting Russian envoy to Tripoli said the Libyan leadership had reiterated that Gaddafi's departure was a "red line" that could not be crossed.
Mikhail Margelov, President Dmitry Medvedev's special representative for Africa, told reporters in Tunis on Friday said Mahmoudi had told him representatives of Gaddafi's government had met rebels in Norway, Germany and France.
Bernard Valero, spokesman for France's foreign ministry, said he could not confirm that talks had taken place between representatives for the rebels and Gaddafi's regime in Paris, adding that France did not initiate any encounter.
Gaddafi has called the rebels "rats" and says NATO's campaign is colonial aggression to steal Libya's oil.
(Additional Reporting by Nick Carey in Tripoli, Nick Vincour in Paris, David Brunnstromin Paris, Tarek Amara in Tunis, Souhail Karam Rabat, Maria Golovnina in Benghazi,Sami Aboudi in Cairo; Writing by John Irish; editing by Matthew Jones)