* Anti-Gaddafi forces attack town near Tunisian border
* 14 rebels killed near Zlitan
* Mystery surrounds killing of rebel military chief
(Updates toll, British comment, Germany expels diplomat)
By Rania El Gamal
BENGHAZI, July 31 (Reuters) - Rebel forces fought gunmen loyal to Muammar Gaddafi in eastern Libya on Sunday in the latest incident to undermine the insurgents’ grip in territory they hold.
The clashes renewed opposition fears that Gaddafi’s agents had infiltrated the area, days after the mysterious killing of the rebel military commander.
The assassination of General Abdel Fattah Younes, apparently by gunmen on his own side, has hurt the opposition just as it was winning broader international recognition and making gains against Gaddafi’s forces in the Western Mountains and elsewhere.
Rebel spokesman Mahmoud Shammam said clashes had broken out when rebel forces attacked a militia that had helped some 300 Gaddafi loyalists break out of jail near Benghazi on Friday.
At least six rebels were killed in the fighting with the militia, whose members appeared to be experienced and armed with machineguns, rocket-propelled grenades and explosives.
Inside the barracks where they were holed up, rebels found more than 400 weapons, Libya’s green flag and photos of Gaddafi.
“At 8 a.m., the barracks was brought under control. Thirty men surrendered and we took their weapons,” Shammam told reporters. “We consider them members of the Fifth Column.”
The clashes reflect growing fears within the opposition that Gaddafi loyalists are exploiting the lawlessness that prevails in the east, which is awash with weapons and armed gangs, some secular or Islamist rebels, some vigilantes and some criminals.
The fighting took place as speculation swirls over the murky circumstances of Younes’ death. The 67-year-old general’s record as Gaddafi’s interior minister before his defection in February, made him the target of suspicion among many in the opposition.
Some Libyans suspect his execution was ordered by rebel leaders for treason, many say he was killed by Gaddafi spies, and others suggest a rebel splinter group had acted alone.
In an apparent effort to avert a feud, rebels named Suleiman al-Obeidi, a member of Younes’ tribe, as acting military chief.
Whatever the truth, the infighting among militias in Libya’s east deepens concerns among the rebels’ Western backers, keen to see them prevail in a five-month-old civil war but frustrated by their lack of unity and worried over Islamist influence.
Keeping up diplomatic pressure on Gaddafi, Britain said on Sunday it would take part in the NATO air campaign for as long as it took and Germany expelled a Libyan diplomat.
The rebels, who rose up against Gaddafi in February, have seized swathes of the country but are poorly equipped and still far from ousting him, despite support from NATO airstrikes.
On Sunday, rebel tanks pounded Gaddafi troops in Tiji, some 200 km (125 miles) southwest of Tripoli, inching one km closer to the last government stronghold in the Western Mountains.
“We are going to take Tiji, I know it. And that will clear the way for us to head to Tripoli eventually,” said fighter Naji Shayboukh, who was holding a home-made rocket-launcher.
About 14 rebels were killed and more than 20 wounded, hospital sources said, in a second day of heavy fighting on the front near Zlitan, some 160 km (100 miles) east of Tripoli and the largest town between rebel-held Misrata and the capital.
In the past 48 hours, rebels have advanced about 3 km towards Zlitan but have yet to solidify their gains.
Television footage obtained by Reuters showed what appeared to be buildings in in Zlitan’s eastern suburb of Zdou. The footage also showed heavy fighting, with rebels using machine guns against Gaddafi’s troops.
“We have advanced well and God willing we will be in Zlitan soon,” said Ibrahim Buwathi, 24, who had a shrapnel wound in his shoulder and was awaiting treatment at the hospital.
About 20 explosions rocked the nearby rebel-held city of Misrata overnight in an apparent attack by Gaddafi loyalists.
Libyan rebels also said they had moved closer to Brega, and were now positioned 5 to 7 km from the east of the oil town.
Fighting at Brega had slowed over the past two weeks as the rebels struggled to defuse hundreds of thousands of mines planted by Gaddafi’s forces.
Rebels said they planned to advance soon on Brega, where some 3,000 heavily armed government troops remain positioned.
The longer the war drags on, the further eastern Libya appears to slip into lawlessness, raising questions about what kind of Libya could emerge if Gaddafi goes.
Many rebels had been uncomfortable working under Younes, a man who had been so close to Gaddafi for 41 years, and rebel sources said on Thursday he had been recalled over suspicions he or his family were secretly in contact with Gaddafi.
Government spokesman Moussa Ibrahim said Younes had been killed by Fawzi Bu Kitf, head of the Union of Revolutionary Forces, a federation of armed rebel groups that works with the Benghazi-based Transitional National Council.
Moussa also accused Ismail al-Sallabi, head of the Feb 17 Brigade, an armed group that falls under the Union umbrella, of involvement in the assassination.
Both have said the had no knowledge of plans to assassinate Younes and that his killers had acted alone.
Rebel sources told Reuters that a field commander called Mustafa al-Rubh, from an armed group called the Okbah Ibn Nafih Brigade, had been sent to the front at Brega to arrest Younes and bring him back to Benghazi for questioning by three judges.
Rubh, who is not under arrest but is being questioned over Younes’ death, said he had taken the military leader to a location outside of Benghazi, where he was due to hand him over.
From here, no one appears able to explain what happened.
Some rebel sources said another militia, called Obaida Ibn Jarrah, intervened, took Younes by force and killed him.
( Additional reporting by Michael Georgy in Hawamid; Mussab Al-Khairalla and Ayman al-Sahili in Misrata; Missy Ryan in Tripoli; Joseph Nasr in Berlin; Souhail Karam in Rabat; writing by Lin Noueihed; Editing by Angus MacSwan)