* Residents of Tripoli district report protest
* UN official says food shortages "a time bomb"
* Rebels clash with Gaddafi forces near Misrata
* Libyan TV says NATO air strikes hit Tripoli
(Recasts, adds death toll, Libyan govt briefing, Malta)
By Peter Graff
TRIPOLI, May 31 (Reuters) - Libyan leader Muammar Gaddafi has made clear that he will not step down, despite the first big protest against him in the capital in months and a U.N. warning on Tuesday that his government was running out of food.
Gaddafi is emphatic he will not leave Libya, South African President Jacob Zuma said after talks with the Libyan leader, dashing prospects for a negotiated end to the conflict.
Zuma was in Tripoli on Monday to try to revive an African "roadmap" for ending the conflict, which started in February with an uprising against Gaddafi and has since turned into a war with thousands of people killed.
Libyan rebels and NATO have set Gaddafi's departure as the main condition for any ceasefire. With Gaddafi's refusal to leave, the talks with Zuma produced no breakthrough.
But new questions emerged over how long Gaddafi could hold on after a senior United Nations aid official said shortages of food and medicine in areas of Libya controlled by Gaddafi amounted to a "time bomb."
Panos Moumtzis, the U.N. humanitarian coordinator for Libya, told Reuters in Tripoli that some food stocks in areas under Gaddafi's control were likely to last only weeks.
"I don't think there's any famine, malnutrition. But the longer the conflict lasts the more the food stocks supplies are going to be depleted, and it's a matter of weeks before the country reaches a critical situation," Moumtzis said in an interview.
"The food and the medical supplies is a little bit like a time bomb. At the moment it's under control and it's ok. But if this goes on for quite some time, this will become a major issue," he said.
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PROTEST IN TRIPOLI
In another development that cast doubt on Gaddafi's assertions that he is in control, witnesses in the Souq al-Juma suburb of Tripoli said a large anti-government protest took place there on Monday. Their accounts lent weight to rebel claims that opposition to Gaddafi was stirring in the capital.
The protest, apparently the biggest confirmed protest inside Tripoli since Western forces began bombing the country in March, was broken up by security forces firing weapons, residents said.
Asked about the incident at a news conference on Tuesday, government spokesman Moussa Ibrahim said: "I have heard of the event. I did not have enough time to get information."
Gaddafi's officials had earlier denied that a large anti-government demonstration took place on Monday.
Large scale demonstrations in Tripoli have not taken place since protests were crushed by the security forces in February.
Activists had released a video on Monday which they said showed hundreds of demonstrators attending a funeral in Souq al-Juma earlier that day for two slain protesters.
State television broadcasts daily rallies in support of Gaddafi and many people in Tripoli tell foreign journalists that they back the Libyan leader.
In conversations with a group of foreign correspondents, several residents reported nightly armed clashes in the area between people opposed to the government and security forces.
Now in its fourth month, Libya's conflict is deadlocked on the ground, with anti-Gaddafi rebels unable to break out of their strongholds and advance towards Tripoli, where Gaddafi appears to be firmly entrenched.
Rebels control the east of Libya around the city of Benghazi, Libya's third-biggest city Misrata, and a mountain range stretching from the town of Zintan, 150 km (95 miles) south of Tripoli, towards the border with Tunisia.
Gaddafi, who has been in power for 41 years, says his forces are fighting armed criminal gangs and al Qaeda militants and portrays the NATO intervention as an act of colonial aggression aimed at grabbing Libya's ample oil reserves.
His government says NATO's bombing campaign has killed 718 Libyan civilians and wounded 4,067, including 433 seriously.
NATO has denied killing large numbers of civilians, and foreign reporters in Tripoli have not been shown evidence of large numbers of civilian casualties.
Asked why the authorities had not shown large numbers of casualties to foreign media, Ibrahim said casualties had not been concentrated near Tripoli but scattered across the country.
Two large explosions were heard in the Libyan capital on Tuesday but it was not immediately clear where the bombs fell.
Speaking in the main rebel stronghold of Benghazi where he was opening a consulate, Italian Foreign Minister Franco Frattini said he had pledged an aid package for the rebels worth hundreds of millions of euros.
"I think the Gaddafi regime is over and I firmly believe that it is over for a simple reason: we are talking about a person whose closest friends are defecting. He lost his legitimacy in Libya," Frattini said.
MORE CLASHES NEAR MISRATA
Western powers have said they expect Gaddafi will be forced out by a process of attrition as air strikes, defections from his entourage and shortages take their toll.
A Reuters photographer in Misrata said there was heavy fighting in the suburb of Dafniyah, in the west of the city, where the front line is now located after rebel fighters drove pro-Gaddafi forces out of the city.
Speaking from a field hospital near the front line, she quoted medical workers as saying one person had been killed and 29 people had been injured so far on Tuesday.
There were reports too of clashes between rebels and forces loyal to Gaddafi in the Western mountains.
A rebel spokesman in the town of Zintan told Reuters by telephone: "Fighting took place last night in (the village of) Rayayna, east of Zintan ... It continued until the early hours of this morning. Both sides used mortars."
Malta recognised the Libyan rebel National Transitional Council as the only legitimate point of dialogue between Malta and Libya on Tuesday and said it would send a delegation to the rebel capital of Benghazi, Al Jazeera television reported. (Additional reporting by Hamid Ould Ahmed in Algiers, Zohra Bensemra in Misrata, Matt Robinson in Zintan, Sherine El Madany in Benghazi, Libya, and Marius Bosch in Johannesburg; Writing by Christian Lowe and William Maclean; Editing by Maria Golovnina and Lin Noueihed)