Two United Nations agencies are calling on European countries to resume migrant rescue operations in the Mediterranean and to stop returns to Libya.

The appeal, issued late Thursday, came in the wake of a deadly airstrike on a Libyan migrant detention centre and continued tensions between Italy, Malta and migrant rescue charities.

"In the past European state vessels conducting search and rescue operations saved thousands of lives," UN Refugee Agency UNHCR and the International Organization for Migration (IOM) said in a statement.

"They should resume this vital work," they add.

They also say that charity vessels "have played a similarly crucial role on the Mediterranean and must not be penalized for saving lives at sea."

The EU's Sophia mission stopped naval patrols in Mediterranean in March due to disagreements on how rescued migrants should be divided between EU member states.

Italy's far-right Interior Minister Matteo Salvini had previously rejected the existing arrangement whereby all those rescued were taken to Italian ports.

According to the UNHCR and IOM, "every effort should be taken to prevent people rescued on the Mediterranean from being disembarked in Libya, which cannot be considered a safe port."

In Libya, where an armed conflict is raging around Tripoli, a July 3 bombing on a detention centre east of the capital killed more than 50 migrants.

The UN agencies say all the 5,600 migrants detained in Libya should be freed and guaranteed protection elsewhere in Libya, or repatriated, or resettled to third countries.

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