* Clashes around Zawiyah, fighting gets closer to Tripoli
* Fighting near Tunisian border to control key supply route
* 21 rebels, six soldiers killed on two eastern fronts
* Government renews charges of NATO massacre
(Adds fighting near Tunisian border)
By Martin Veal and Missy Ryan
NEAR ZAWIYAH/TRIPOLI, Libya, Aug 13 (Reuters) - Libyan government forces and rebels clashed around the western town of Zawiyah on Saturday as the insurgents tried to push closer to the capital Tripoli.
Reporters heard gunfire and skirmishing in the coastal town, about 50 km (30 miles) west of Tripoli. The highway from the capital to the Tunisian border was blocked there.
The government confirmed fighting in the area but said a rebel attempt to capture Zawiyah had been beaten back.
Zawiyah is "absolutely under our control," government spokesman Moussa Ibrahim told reporters in Tripoli. "A very small group of rebels tried to move into the south of Zawiyah but they were stopped easily because of our armed forces," he said.
But a rebel spokesman, Mohammed Ezzawi, speaking from inside Zawiyah, said the rebel force was about 800 metres from Martyrs' Square in the city centre.
"The Gaddafi Brigade occupy the eastern part of the main road while we are on the western side. There has been an intense exchange of fire on this road, which links Tripoli to Tunisia," he told Reuters by telephone.
"If we manage to take Zawiyah we will be blocking this road and it will mean the death of Gaddafi," he said, predicting the town would fall by Sunday.
If rebels take Zawiyah, an oil refinery town close to Tripoli, it would deliver a psychological blow to Gaddafi's supporters. It would also leave the capital under partial siege because the main supply route to Tunisia would be cut and Tripoli would be starved of fuel, already in short supply.
Tunisians close to the border later told Reuters that rebels were clashing with Gaddafi's forces a few kilometres from the main Ras Jdir border crossing, which also controls the vital road carrying supplies to Tripoli.
A businessman called Ali, who trades with Libya, said there were clashes at Abu Kammash, an industrial town on the Mediterranean coast about 10 km from Ras Jdir.
A second businessman from the border area, who did not want to be named, said: "There are heavy clashes going on ... to try to control the Ras Jdir crossing." A third source, at the crossing, said the military had brought up tanks to protect the checkpoint.
CELEBRATIONS, PARTIAL CONTROL
Rebel fighters returning south to Bir al-Ghanam from Zawiyah told Reuters the rebel force was in the city centre but not in complete control.
"We took over the centre of Zawiyah about an hour ago. There were mercenaries there. The fighting lasted about 30 minutes and then they ran away," said a rebel fighter named Ahmed.
He said people had come out in to the streets to celebrate.
A second rebel, Abdelsalam, said: "We're in control of the centre. Some Gaddafi troops have fled to Tripoli, some are left over, and there are also mercenaries in the town. So we don't have complete control yet."
Dr Asim Shaybee, at a field hospital at Bir Ayyad gate south of the fighting, said four rebels were killed by an accidental NATO airstrike on a rebel tank at Zawiyah. Several rebels were wounded,"one of them ... shot in the head by a sniper," he said.
According to government spokesman Ibrahim, fewer than 100 rebels entered the city from the south and tried to join 50 rebels within the city, but they had been "dealt with".
Government forces were still fighting the rebels inside the city, he said, adding that it was "not an advance but a skirmish, a suicide mission".
The source inside Zawiyah said the rebel advance was helped by a NATO strike at Nattafah 15 km south of the city. "There was a very large number of Gaddafi forces in that region," he said.
Rebels trying to overthrow Gaddafi hope to capture Zawiyah and cut off his stronghold in the capital from access to the outside world by severing the coastal highway, which has been a lifeline for the embattled government.
They advanced north to within 25 km (15 miles) of Zawiyah earlier in the day, following what they said was a six-hour battle which pushed the front line closer to Tripoli.
CASUALTIES HIGHER IN EAST
In other fighting on two fronts well to the east of Tripoli, at Brega and near Misrata, at least 21 rebels and six soldiers were killed over the past two days, with some 50 rebels wounded. Neither side claimed major advances in the past 24 hours.
Libya's state news agency said a NATO air strike had killed six "martyrs" in Brega, and the alliance confirmed it had attacked two armoured vehicles there.
Judging by impact craters, wrecked buildings and burnt-out tanks, NATO warplanes have also bombed government military targets on the route of the western rebel advance to Zawiyah over the past week, providing close air support.
Zawiyah is the home town of many of the rebels battling on the western front and has staged two uprisings against Gaddafi since the nationwide revolt broke out in February.
On Libya's most easterly front, at least 21 rebels and government soldiers were killed in fighting for the oil terminal of Brega in the past two days, hospital workers said.
A volunteer at the hospital in Ajdabiyah, where fighters wounded in Brega are taken, said 15 rebel fighters had been killed and about 50 wounded. He said the bodies of six government soldiers were brought in on Friday.
In fighting around a second eastern front in Misrata, much closer to Tripoli, at least six rebels were killed in the past 24 hours, rebel sources said.
In Misrata, a port on the Mediterranean under rebel control for months, six rebel fighters were killed in fighting on Friday. There was no word on government casualties.
Three rebels were killed west of the city in fighting for Zlitan, west of Misrata. Further south, three more died in battles with Gaddafi's forces in the town of Tawargha.
(Additional reporting by Tarek Amara in Tunisia, Missy Ryan in Tripoli, Robert Birsel in Benghazi, Joseph Nasr in Berlin; Souhail Kamar is Rabat; Michael Georgy in Bir al-Ghanam. Writing by Douglas Hamilton; Editing by Tim Pearce)