(Adds daughter gives birth in Algeria, commander estimates 50,000 killed)
* Interim leader tells Gaddafi forces to surrender by Saturday
* Gaddafi wife, three children in Algeria, daughter gives birth
* Anti-Gaddafi commander says war has killed some 50,000
* Tripoli still lacks water, power on eve of Muslim feast
* Gaddafi son's bodyguard says leader left Tripoli on Friday
By Samia Nakhoul and Alexander Dziadosz
TRIPOLI/BENGHAZI, Aug 30 (Reuters) - Libya's interim leader gave forces loyal to deposed ruler Muammar Gaddafi a four-day deadline on Tuesday to surrender towns they still control or face a bloody end to a war that the new leadership said has so far killed 50,000 people.
As the hunt for Gaddafi himself goes on, Libyan officials accused Algeria of an act of aggression for giving refuge to his fleeing wife and three of his children, as well as, it turned out, to a new grand-daughter, born on Tuesday.
Algeria's Foreign Ministry said Gaddafi's wife Safia, and his sons Hannibal and Mohammed had entered Algeria on Monday morning, along with their children. His pregnant daughter Aisha was also among the party and she gave birth within a day to a girl, a source close to Algeria's health ministry said.
The incident stirred a diplomatic row as Libya's interim council works to consolidate its authority and capture places still loyal to Gaddafi, notably the coastal city of Sirte.
Anti-Gaddafi forces have converged on Sirte from east and west, but have stopped short of an all-out assault in hopes of arranging a negotiated surrender of Gaddafi's birth-place.
"By Saturday, if there are no peaceful indications for implementing this, we will decide this matter militarily. We do not wish to do so but we cannot wait longer," Mustafa Abdel Jalil, head of Libya's interim council, told a news conference.
In Benghazi, headquarters of the anti-Gaddafi National Transtional Council during the uprising, military spokesman Colonel Ahmed Bani said the negotiations over Sirte involved tribal elders, not Gaddafi loyalists themselves.
The loyalists were thwarting the desire of most civilians to join the liberated areas, he said.
"Zero hour is quickly approaching," Bani said.
At forward positions of NTC forces, on the main coastal road some 100 km (60 miles) west of Sirte, a Reuters correspondent saw little sign of military action on Tuesday.
Six months of fighting has left some 50,000 dead, one anti-Gaddafi commander said, an estimate that was hard to verify and which, he said, included many people who had gone missing.
"About 50,000 people were killed since the start of the uprising," Colonel Hisham Buhagiar, commander of the anti-Gaddafi troops who advanced on Tripoli out of the Western Mountains, known as Jebel Nafusa, told Reuters.
"In Misrata and Zlitan between 15,000 and 17,000 were killed and Jebel Nafusa took a lot of casualties. We liberated about 28,000 prisoners. We presume that all those missing are dead," he said. "Then there was Ajdabiyah, Brega. Many people were killed there too."
Gaddafi's whereabouts have been unknown since his foes seized his Tripoli compound on Aug. 23, ending his 42-year rule after a six-month revolt backed by NATO and some Arab states.
Britain's Sky News, citing a young bodyguard of Gaddafi's son Khamis, said the leader had stayed in Tripoli until Friday when he left for the southern desert town of Sabha.
It quoted the captured 17-year-old as saying Gaddafi met Khamis, a feared military commander, at around 1:30 p.m. on Friday in a Tripoli compound that was under heavy rebel fire. Gaddafi had arrived by car and was soon joined by Aisha.
After a short meeting, they boarded four-wheel drive vehicles and left, the bodyguard told a Sky reporter, adding that his officer had told him: "They're going to Sabha."
Along with Sirte, Sabha is one of the main remaining bastions of pro-Gaddafi forces.
A NATO spokesman said the alliance, which has kept up a five-month bombing campaign, was targeting the city's approaches.
"Our main area of attention is a corridor... (leading up) to the eastern edge of Sirte," Colonel Roland Lavoie said.
Some anti-Gaddafi officers have reported that Khamis Gaddafi and former intelligence chief Abdullah al-Senussi were both killed in a clash on Saturday. This has not been confirmed and the NATO spokesman said he had no word on Khamis's fate.
More NTC forces were heading for Bani Walid, a Gaddafi tribal stronghold 150 km (95 miles) southeast of Tripoli.
"Three units were sent from Misrata toward Bani Walid this morning ... Our fighters are now 30 km from Bani Walid," said Mohammed Jamal, a fighter at a checkpoint on the road to the town. "Hopefully Bani Walid will also be liberated soon. Right now there are still many Gaddafi supporters there."
An NTC spokesman said it would seek to extradite Gaddafi's relatives from Algeria, which is alone among Libya's neighbours in not recognising the de facto government.
Nearly 60 countries have acknowledged the NTC as Libya's legitimate authority. Russia, China, India, South Africa and Brazil are among those which have so far withheld recognition.
Algeria's acceptance of Gaddafi's wife and offspring angered Libyan leaders, who want the ousted autocrat and his entourage to face justice for years of repressive rule.
Abdel Jalil, the NTC chairman, who was previously Gaddafi's justice minister, called on Algeria to hand over any of the former leader's sons on its wanted list. He said he expected the fugitives to move on from Algeria before long.
Algeria, which previously opposed sanctions and a no-fly zone against Gaddafi, has an authoritarian government which is anxious about Arab revolts lapping near its borders.
"I would argue the Algerian regime is making a major blunder, miscalculating monstrously," Fawaz Gerges, an analyst at the London School of Economics, told the BBC.
"The Algerian regime itself is not immune from the revolutionary momentum taking place in the Arab world."
A visit to a Tripoli beach compound used by Gaddafi's children and members of his elite revealed a life of opulence and privilege that many Libyans could barely dream of.
Saadi Gaddafi's chalet was strewn with designer clothes, including some unworn suits, and about 100 pairs of shoes. Aisha's house boasted 13 bedrooms and gold-plated cutlery.
Anti-Gaddafi fighters now sleep in the bedrooms of their former rulers, whose gated compound has tennis courts, football pitches and dining centres, along with magnificent sea views.
Many Libyans were overjoyed at the fall of Gaddafi, which followed that of longtime rulers in Egypt and Tunisia earlier this year, but have been chilled by evidence of mass killings in Tripoli as his forces fought losing battles with rebels.
A week after Gaddafi's fall, Tripoli's two million people remain without running water or electricity. Banks, pharmacies and many other shops are still closed. The stench of garbage and sewage still pervades the city, despite clean-up efforts.
A council spokesman said a pumping station for Tripoli's water supply that lies in a pro-Gaddafi area had been damaged and could not be reached for repair.
However, a report by the European Union's humanitarian office (ECHO), said pro-Gaddafi forces in Sirte had cut two-thirds of the water supply to Tripoli, most of which comes from the "Great Man-made River", a huge project built under Gaddafi that pumps out water from under the Sahara desert.
In the port city of Misrata, scene of heavy fighting earlier in the conflict, security forces were holding 332 former Gaddafi fighters in a school, where the captives sat on mattresses in the classrooms, some reading the Koran.
There was no evidence the men had been mistreated