Algeria's UN envoy has defended his country's decision to grant refuge to the wife and three children of fugitive Libyan leader leader Muammar Gaddafi. Mourad Benmehidi told the BBC that in the desert region there was a "holy rule of hospitality". A rebel spokesman called the move an "act of aggression against the Libyan people" and said they would use all legal means to compel them to return. Meanwhile, more details have emerged about recent mass killings in Libya. According to the NTC's Justice Minister Mr Mohammed al-Alagi, four mass graves have been discovered across Libya - including one at Ain Zara in south-east Tripoli, situated behind the barracks of the so-called Khamis Brigade, whose commander was Col Gaddafi's son, Khamis. Scores of people - mostly army officers who refused to fight for Col Gaddafi in recent weeks - are said to have been killed here, and a rights group says it is collecting evidence to pass to the International Criminal Court (ICC) for a possible war crimes prosecution. Libyan rebels seized most of the capital Tripoli on 21 August, but fighting still goes on in pockets of the country - notably around Col Gaddafi's hometown of Sirte. Col Gaddafi's own whereabouts are unknown, and are the subject of intense speculation, with rumours variously placing him in Sirte, in regime-controlled Bani Walid south-east of Tripoli, and in Tripoli itself. 'Act of aggression' A foreign ministry statement said Col Gaddafi's wife Safia, daughter Aisha and sons Muhammad and Hannibal crossed the border between Libya and Algeria at 0845 local time (0745 GMT) on Monday. But the BBC's Jon Leyne in the eastern Libyan city of Benghazi said first word of such a move had already come from Libyan rebel headquarters two days ago, and that at the time, Algerian authorities denied that a convoy of six heavily armoured vehicles had crossed the border. Algeria is an obvious refuge for the Gaddafi family as the two countries have a long border and the Algerian government has still not recognised the rebel National Transitional Council (NTC), our correspondent says. The confirmation of the Gaddafis' escape caused fury at the rebel NTC, where spokesman Mahmoud Shamman said: "This would be an act of aggression against the Libyan people and against the wishes of the Libyan people. "We will use all legal means to seek the return of these criminals and to bring them to justice in Libya." In an interview with news agency Reuters, Mr Shamman added: "We are warning anybody not to shelter Gaddafi and his sons. We are going after them... to find them and arrest them." "We have heard that Algeria will harbour them till they go to another country. They are trying to go to another country, possibly an east European country," he said. But in an interview with the BBC World Service, Ambassador Benmehidi insisted that his country had a duty to provide assistance, "and in fact in many parts of the Sahara region it's mandatory by law to provide assistance to anyone in the desert". He said Algiers had quickly informed both the UN secretary general and the Libyan rebels of the arrival of the Gaddafis - pointing out that none of those who had entered Algeria were the subject of arrest warrants from the ICC. Algeria has signed the Rome Statute of the International Criminal Court, but has not ratified the treaty. He pledged his country would fulfil its international obligations. Meanwhile rebels said they had uncovered four mass graves in recent days. The BBC's Andrew Hosken was the latest BBC reporter to visit one of the sites in south-east Tripoli, near the barracks of the army brigade headed by Col Gaddafi's son Khamis. The barn was discovered full of charred corpses, believed to be those of army officers who refused to fight for Col Gaddafi. One witness told our correspondent he had personally seen between 150 and 160 people shot inside and outside the barn, and work is under way to try to uncover bodies believed to have been buried around the barn. The New York-based group Human Rights Watch has representatives at the scene, collecting data and witness information, in the hope that the case could go to the ICC. On Monday, rebel Col Al-Mahdi Al-Haragi was quoted as saying Khamis, who has led a feared army unit blamed for a massacre last week, had died after being badly wounded in a clash. Rebel military spokesman Ahmed Bani said bodies in a convoy destroyed during the clash were burnt beyond recognition, but captured soldiers said they were Khamis' bodyguards. He also said Col Gaddafi's brother-in-law and the head of his intelligence services, Abdullah al-Sanussi, was "almost certainly killed" in the same confrontation. Leaked UN document There were reports earlier in the conflict that Khamis had been killed in an air strike, though those reports were later questioned. Claims earlier this month that rebels had detained Col Gaddafi's most prominent son, Saif al-Islam, turned out not to be true. Rebel fighters have been pushing towards Sirte, and earlier took the small town of Nofilia on their way to the city. The rebels say they think Col Gaddafi himself may still be in the Tripoli area, says our correspondent. Meanwhile, a leaked document that appears to outline UN proposals for post-conflict Libya calls for up to 200 military observers and 190 UN police to help stabilise the country. The deployment would follow a UN mission with a core staff of 61 civilians for an initial three month period, according to the report on the website Inner City Press. Any such plan would be implemented only if requested by the Libyan transitional authorities and approved by the Security Council, it said.