The United Nations voiced "extreme alarm" Tuesday at reports of "atrocious human rights violations" in Libya, including mass summary executions apparently carried out by Colonel Muammar al-Qadhafi's forces in the last few days before they lost Tripoli, the capital.

Displaced people in eastern Libya outside the town of Ajdabiya [UN]

"We are also deeply concerned about reports that there are still thousands of people unaccounted for who were arrested or taken prisoner by Qadhafi security forces either earlier in the conflict, or before it even started," UN High Commissioner for Human Rights (OHCHR) spokesperson Rupert Colville told a news briefing in Geneva.

"Given the gruesome discoveries that have taken place over the past few days, there is good reason to be extremely concerned for their safety. We urge any members of the former regime in a position to reveal where prisoners have been held to do so, before more lives are lost."

Don't miss

• Gaddafi's youngest son killed in clash

? In pictures: Gaddafi family

? Gaddafi to be handed over if he enters Algeria

Secretary-General Ban Ki-moon appealed to the Security Council for quick decisive action now that the National Transition Council (NTC) appears to be largely in control of Tripoli and other cities, affording hope for a quick conclusion to the conflict and an end to the suffering of Libya's people.

"My aim is to get UN personnel on the ground absolutely as quickly as possible, under a robust Security Council mandate," he said in a briefing to the 15-member body, referring to his plans to speedily set up a UN mission in Libya to help the new authorities deliver immediate emergency aid and provide a democratic transition.

"I cannot overstate the urgency of this moment. Time is of the essence. The people of Libya are looking to the international community for help," he added, noting that water supplies are critically short, with 60 per cent of Tripoli's 3-million-strong population without water and sanitation, and stressing the urgent need to provide sufficient fuel and spare parts to avoid further shutdowns of the water system.

"I appeal to the Council to continue to be responsive to the requests of the transitional authorities for funding," he said. "Our most important job will be to ensure that multilateral, regional and bilateral efforts are complementary and correspond to Libyan wishes. In turn, this will require Libya's transitional authorities to provide clear priorities – short term and longer term."

Mr. Ban also referred to the growing evidence of summary executions, torture and human rights violations. "These will be looked into by the International Commission of Inquiry on Libya, which is expected to submit its first oral update to the Human Rights Council on 19 September," he said.

Meanwhile, the UN World Food Programme (WFP) is scaling up its logistical support for the humanitarian relief effort in the strife-torn North African country, sending in urgent supplies of food, water, medicine and fuel to Tripoli, the coastal areas and the Nafusa mountain region where rebels, recognized by much of the international community, have ousted pro-Qadhafi forces.

The aid includes 600 tons of staple food commodities – including wheat flour, pasta, vegetable oil and tomato paste – for distribution by the Libyan Red Crescent for more than 35,500 conflict-affected and displaced people for one month.

A WFP vessel is carrying 500,000 litres of water on behalf of the UN Children's Fund (UNICEF) from Malta to Tripoli and, at the request of the NTC, is looking to procure 250,000 tons of petrol to cover immediate life-saving needs for one month.

Fuel supplies have been disrupted by the fighting, and water and electricity depend on fuel-run generators. Fuel is also required for hospitals, ambulances and vehicles to distribute critically needed medicines, food, water, and other supplies.

Last week WFP delivered 500 tons of food to the Gheryan area in north-western Libya, where an estimated 200,000 people are in need of food, and has completed delivery of another 495 tons of mixed food to the Zliten region, enough to feed 15,000 people for one month.

On Thursday Mr. Ban will attend the International Conference for Support of Libya convened in Paris, accompanied by his Special Advisor for Post-Conflict Planning for Libya, Ian Martin, and his Special Envoy, Abdel Elah Al-Khatib, as well as Under Secretary-General for Political Affairs B. Lynn Pascoe.

Speaking to reporters after the Council's closed-door session on Libya, Mr. Martin said there was a strong wish in the Council and within the NTC that the UN should play the key role in supporting overall international assistance to Libya.

"I am very pleased to say there was a very strong endorsement in the Council of the approach, that was reflected in the Secretary-General's statement, that we are taking to planning the support of the United Nations to Libya as the conflict comes to an end," he said.

The Council also welcomed "the fact that the United Nations system had engaged in preparatory work that puts us in a strong position now to be ready to respond to Libyan requests," he added.