South Sudan released seven rebel detainees Thursday but vowed to put on trial key leaders accused of launching weeks of fighting, a move likely to threaten a fragile ceasefire. Both sides implemented the ceasefire last Friday, but combat has only eased, not ended, with reports of continuing clashes and a worsening humanitarian crisis that has left thousands dead and forced almost 800,000 to flee their homes. Four leaders remain in custody in South Sudan, facing trial for attempting to topple President Salva Kiir after fighting broke out in the capital Juba on December 15. Kiir accused his sacked deputy Riek Machar and other former officials of fomenting a coup against his government. Eleven ex-officials were arrested, while Machar -- who denied any coup plot -- fled. Fighting quickly spread across the country. Aid groups say up to 10,000 people have been killed in the conflict, although many fear more may have died. United Nations aid chief Valerie Amos wrapped up a three-day visit Wednesday to the war-torn country, where she saw the results of over six weeks of bloodshed, with horrific atrocities reported to have been committed by both sides. "The future of South Sudan rests on all the people being able to work together," she said, after a tour in which she saw food stores looted of tonnes of food aid, in devastated towns where workers were still burying those recently killed in the fighting. The fighting has seen waves of brutal revenge attacks, as fighters and ethnic militia use the violence to loot and settle old scores. South Sudan's Justice Minister Paulino Wanawila said Tuesday that the four men in detention will face trial while three others, including Machar, will face justice if caught. "If someone violates the law you don't go and torture that person, you prosecute that person according to the law," he said. 'Things may get worse before they get better' But the release of all the prisoners has been a key demand of the rebels, and Kenya's foreign ministry said it was "still negotiating for the release of the remaining four." The United States urged Juba to free the remaining detainees to help boost political reconciliation, as it welcomed the releases as an "important step towards an inclusive political dialogue". John Luk Jok, a former justice minister, spoke on behalf of those released, who appeared in apparent good health. "We don't feel bitter, we only feel sad that the crisis in our country is happening just after our independence," he said. "We don't see our president as our enemy." Many fear the conflict has slid out of the control of political leaders, with ethnic violence and revenge attacks between the Dinka people of Kiir and the Nuer of Machar, the country's two largest groups. Over 76,000 civilians are still sheltering inside UN peacekeeping bases. Government and opposition rebels are still fighting for control in key areas, with the United Nations calling the situation "fragile". Toby Lanzer, UN aid chief in South Sudan, warned that they were in a "race against time" to support those affected, with the rainy season looming and thousands living outside. "I think the very sad reality for people, for civilians in South Sudan, is that things may get worse before they get better," he said, but saying it was "never too early" to start reconciliation. Clashes were reported Wednesday in the oil-producing Unity state, close to the settlement of Leer, Machar's home region. Aid workers have said they were struggling to cope with those needing support. "The crisis today is already very severe, you have hundreds of thousands of people displaced, and many, many wounded, who have come to our facilities," said Arjan Hehenkamp, from Medecins sans Frontieres (Doctors Without Borders, MSF). "We face an emergency today, for many more than only the displaced, and we will continue to see that for the next six months and probably into the next year." MSF has pulled out staff from its clinic in Leer fearing violence. Both sides say the other has already broken the ceasefire, but insist they are committed to the deal. Kiir has hinted he could use his power to grant amnesties but has said that legal processes must be completed first.