"As a priority we ask that 5,600 refugees and migrants currently held in centres across Libya be freed in an orderly manner and their protection guaranteed," UN refugee agency chief Filippo Grandi and the UN migration agency, Antonio Vittorino said in a joint statement.
"Suffering and risk of human rights abuses continue" in Libya's migrant detention centres, the statement underlined.
Some of the migrants who are being detained could be evacuated to other countries, but "accelerated resettlement" from these countries is needed, said the statement, urging nations to "step forward with more evacuation and resettlement places".
Migrants who wish to return to their home countries should be able to do so, the statement added.
"Detention of those disembarked in Libya after being rescued at sea has to stop. Practical alternatives exist," the statement said.
Migrants released from detention centres should be allowed to live in local communities or in open and "semi-open" safe centres that can be set up along the lines of UNHCR's Gathering and Departure Facility in Tripoli, where migrants can be registered, said the statement.
Over 50 refugees and migrants were killed in an air raid on the Tajoura detention facility east of Tripoli last week, prompting the UNHCR and the International Organisation for Migration to appeal to the European Union to prevent such a tragedy from re-occurring, the statement recalled.
"The international community should consider the protection of the human rights of migrants and refugees a core element of its engagement in Libya," the statement said.
The UN-backed government in Tripoli blamed the deadly Tajoura air strike on eastern warlord Khalifa Haftar's forces.
UN Secretary General Antonio Guterres has urged an independent investigation of the bombing, which he said may amount to a war crime. The UN-backed government in Tripoli blamed the attack on eastern warlord Khalifa Haftar's forces.
The Tajoura detention centre closed on Wednesday, and some 400 survivors of the July 3 attack have been moved to UNCHR's Gathering and Departure Facility, the statement said. The facility is now severely overcrowded and efforts are being made to evacuate those sheltered there, according to the statement.
The statement also appealed for more aid to improve the lot of approximately 50,000 registered refugees and asylum-seekers and estimated 800,000 migrants who are living in Libya, mainly in urban centres.
"More help is required so that living conditions are improved, human rights are better protected, and fewer people end up being driven into the hands of smugglers and human traffickers," it said.
The statement asked for "every effort" to be made to prevent people saved in the Mediterranean from being disembarked in Libya, saying it could not be considered a safe port.
The UNHCR and the IOM also urged European countries to resume migrant search and rescue operations that have saved thousands of lives and called on them not to penalise charity rescue ships (currently the only vessels saving migrants in the Central Mediterranean)
"NGO boats have played a similarly crucial role on the Mediterranean and must not be penalized for saving lives at sea.
Temporary disembarkation schemes should urgently be established by Europe "to share responsibilities", the statement recommended.
"Any assistance and responsibilities assigned to relevant Libyan entities should be made conditional on no one being arbitrarily detained after they have been rescued and guarantees of human rights standards being upheld. Without such guarantees, support should be halted."
( With inputs from IANS )